Add a platform-neutral mvt command

Several MVT commands have nothing to do with the acquisition of one
platform, yet they were reachable only through mvt-ios and mvt-android.
Asking which version is installed or downloading the public indicators
meant picking one of the two platform commands arbitrarily, and each of
those tasks had to be written, documented and maintained twice.

Add a third console script, mvt, hosting the commands which belong to no
platform: version and download-iocs for now, with completion following
in a later commit.

Commands installed in the new mvt.cli_plugins entry-point group are
registered on mvt, and on mvt only, so that a command package chooses
the CLI each of its commands is added to: mvt.ios.cli_plugins for
mvt-ios, mvt.android.cli_plugins for mvt-android and mvt.cli_plugins for
mvt. A command wanted on both platform CLIs is registered in both
platform groups; no group adds a command to every CLI. The
MVT_CUSTOM_COMMANDS variable loads command files and folders into mvt
the way the platform variables already do for mvt-ios and mvt-android.

Run on its own, mvt prints the banner and its help instead of a usage
error. The help text reminds that the forensic analysis of an
acquisition runs through mvt-ios and mvt-android, so that the command
which knows nothing about acquisitions says where they are analysed.

version and download-iocs stay on mvt-ios and mvt-android for now, so
that no documented invocation stops working. They are to be dropped from
the platform CLIs in a later release, once mvt has been available long
enough for the change to be announced.

Unlike mvt-ios and mvt-android, which point at their subpackages, the
console script points at mvt.cli:main and the mvt package re-exports
nothing of it. Importing mvt has to stay cheap and free of side effects:
it is the package plugins import from, and pulling in Click, the CLI and
everything the commands import merely because something imported mvt
would work against that.

While here, give the version command of both platform CLIs the context
settings every other command already has, so that "mvt-ios version -h"
prints its help instead of failing on an unknown option.

tests/test_cli_entry_points.py

@@ -10,10 +10,20 @@ import click
 import pytest
 
 import mvt.android
+import mvt.cli
 import mvt.ios
 from mvt.android.cli import cli as android_cli
 from mvt.android.cli import main as android_main
-from mvt.common.cli_plugins import ANDROID_CLI_PLUGIN_GROUP, IOS_CLI_PLUGIN_GROUP
+from mvt.cli import cli as mvt_cli
+from mvt.cli import main as mvt_main
+from mvt.common.cli_plugins import (
+    ANDROID_CLI_PLUGIN_GROUP,
+    IOS_CLI_PLUGIN_GROUP,
+    MVT_ANDROID_CUSTOM_COMMANDS_ENV,
+    MVT_CUSTOM_COMMANDS_ENV,
+    MVT_IOS_CUSTOM_COMMANDS_ENV,
+    NEUTRAL_CLI_PLUGIN_GROUP,
+)
 from mvt.ios.cli import cli as ios_cli
 from mvt.ios.cli import main as ios_main
 
@@ -39,23 +49,32 @@ def cli():
 """
 
 PROGRAMS = {
-    "mvt-ios": (mvt.ios, ios_cli, IOS_CLI_PLUGIN_GROUP),
-    "mvt-android": (mvt.android, android_cli, ANDROID_CLI_PLUGIN_GROUP),
+    "mvt": (mvt.cli, mvt_cli, NEUTRAL_CLI_PLUGIN_GROUP, MVT_CUSTOM_COMMANDS_ENV),
+    "mvt-ios": (mvt.ios, ios_cli, IOS_CLI_PLUGIN_GROUP, MVT_IOS_CUSTOM_COMMANDS_ENV),
+    "mvt-android": (
+        mvt.android,
+        android_cli,
+        ANDROID_CLI_PLUGIN_GROUP,
+        MVT_ANDROID_CUSTOM_COMMANDS_ENV,
+    ),
 }
 
+CASE_SUMMARY_COMMAND = """
+import click
 
-@pytest.fixture
-def restore_cli_commands():
-    """Undo the plugin registration main() performs on the shared CLI groups."""
-    originals = {
-        program: dict(group.commands) for program, (_, group, _) in PROGRAMS.items()
-    }
-    yield
-    for program, (_, group, _) in PROGRAMS.items():
-        group.commands.clear()
-        group.commands.update(originals[program])
-        if hasattr(group, "_mvt_external_command_sources"):
-            delattr(group, "_mvt_external_command_sources")
+
+@click.command("case-summary")
+def cli():
+    click.echo("case summary ran")
+"""
+
+# The entry-point group of another program, for each program: no group may add
+# its commands to a CLI other than its own.
+OTHER_PROGRAMS_GROUP = {
+    "mvt": IOS_CLI_PLUGIN_GROUP,
+    "mvt-ios": NEUTRAL_CLI_PLUGIN_GROUP,
+    "mvt-android": NEUTRAL_CLI_PLUGIN_GROUP,
+}
 
 
 def _install_fixture_entry_point(monkeypatch, entry_point_group, command):
@@ -92,7 +111,7 @@ def _offline_argv(program, *arguments):
 def test_main_registers_installed_plugins_before_running_the_cli(
     program, monkeypatch, capsys, restore_cli_commands
 ):
-    package, group, entry_point_group = PROGRAMS[program]
+    package, group, entry_point_group, _ = PROGRAMS[program]
 
     @click.command()
     def fixture_command():
@@ -113,7 +132,7 @@ def test_main_registers_installed_plugins_before_running_the_cli(
 def test_main_completes_plugin_command_names(
     program, monkeypatch, capsys, restore_cli_commands
 ):
-    package, _, entry_point_group = PROGRAMS[program]
+    package, _, entry_point_group, _ = PROGRAMS[program]
 
     @click.command()
     def fixture_command():
@@ -136,17 +155,9 @@ def test_main_completes_plugin_command_names(
 def test_main_still_loads_commands_from_a_file(
     program, monkeypatch, capsys, tmp_path, restore_cli_commands
 ):
-    package, _, entry_point_group = PROGRAMS[program]
+    package, _, entry_point_group, _ = PROGRAMS[program]
     command_path = tmp_path / "case_summary.py"
-    command_path.write_text(
-        "import click\n"
-        "\n"
-        "\n"
-        '@click.command("case-summary")\n'
-        "def cli():\n"
-        '    click.echo("case summary ran")\n',
-        encoding="utf-8",
-    )
+    command_path.write_text(CASE_SUMMARY_COMMAND, encoding="utf-8")
     _install_fixture_entry_point(
         monkeypatch, entry_point_group, click.Command("unused")
     )
@@ -163,12 +174,66 @@ def test_main_still_loads_commands_from_a_file(
     assert "case summary ran" in capsys.readouterr().out
 
 
+@pytest.mark.parametrize("program", sorted(PROGRAMS))
+def test_main_loads_commands_from_the_environment_variable(
+    program, monkeypatch, capsys, tmp_path, restore_cli_commands
+):
+    # Each CLI reads its own variable, so a main() reading another CLI's would
+    # go unnoticed without this.
+    package, _, _, environment_variable = PROGRAMS[program]
+    command_path = tmp_path / "case_summary.py"
+    command_path.write_text(CASE_SUMMARY_COMMAND, encoding="utf-8")
+    monkeypatch.setenv(environment_variable, str(command_path))
+    monkeypatch.setattr(sys, "argv", _offline_argv(program, "case-summary"))
+
+    with pytest.raises(SystemExit) as exit_info:
+        package.main()
+
+    assert exit_info.value.code == 0
+    assert "case summary ran" in capsys.readouterr().out
+
+
+@pytest.mark.parametrize("program", sorted(PROGRAMS))
+def test_main_ignores_the_entry_point_groups_of_the_other_programs(
+    program, monkeypatch, capsys, restore_cli_commands
+):
+    package, group, _, _ = PROGRAMS[program]
+    _install_fixture_entry_point(
+        monkeypatch,
+        OTHER_PROGRAMS_GROUP[program],
+        click.Command(FIXTURE_COMMAND_NAME),
+    )
+    monkeypatch.setattr(sys, "argv", _offline_argv(program, "--help"))
+
+    with pytest.raises(SystemExit) as exit_info:
+        package.main()
+
+    assert exit_info.value.code == 0
+    assert FIXTURE_COMMAND_NAME not in group.commands
+    assert FIXTURE_COMMAND_NAME not in capsys.readouterr().out
+
+
 def test_the_console_script_targets_are_importable():
-    # [project.scripts] points at these, so they must stay on the packages.
+    # [project.scripts] points at these, so they must stay where they are.
+    assert mvt.cli.main is mvt_main
     assert mvt.ios.main is ios_main
     assert mvt.android.main is android_main
 
 
+def test_importing_mvt_does_not_import_a_cli(tmp_path):
+    # The mvt package deliberately re-exports nothing of mvt.cli, so that
+    # importing MVT stays cheap and free of side effects.
+    result = run_isolated_python(
+        "import sys\n"
+        "import mvt\n"
+        "print('imported a cli' if 'mvt.cli' in sys.modules else 'imported mvt')\n",
+        home=tmp_path / "home",
+    )
+
+    assert result.returncode == 0, result.stderr
+    assert result.stdout.strip() == "imported mvt"
+
+
 def test_importing_mvt_does_not_run_installed_plugins(tmp_path):
     site_path = write_cli_plugin_distribution(
         tmp_path / "site", IOS_CLI_PLUGIN_GROUP, MARKER_PLUGIN_TEMPLATE