Keep the start of mvt-ios and mvt-android cheap for shell completion

Each console script imports its CLI module before Click can answer a
shell completion request, and the completion scripts run the program on
every keystroke. Importing mvt.ios.cli or mvt.android.cli took ~230 ms,
of which building the command tree needed almost nothing: cli_plugins
imported one constant from module_loader, which pulled in MVTModule, the
indicators, the pydantic settings, requests and rich; the command
implementations pulled in the same, and the iOS CLI imported iOSbackup
(pycryptodome) for decrypt-backup.

The two platform CLI modules now only build the command tree: each
command imports what it runs when it is invoked. cli_plugins owns the
custom command prefix instead of importing it from module_loader, and
exec_or_profile() loads the settings when it runs.

Completion of mvt-ios and mvt-android drops from ~260 ms to ~85 ms per
keystroke on a clean install, and every command starts that much
sooner. A test fails as soon as a CLI module imports the module
machinery again.

tests/test_cli_startup.py

@@ -0,0 +1,28 @@
+# Mobile Verification Toolkit (MVT)
+# Copyright (c) 2021-2026 The MVT Authors.
+# Use of this software is governed by the MVT License 1.1 that can be found at
+#   https://license.mvt.re/1.1/
+
+import pytest
+
+from .plugin_fixtures import run_isolated_python
+
+# Importing a platform CLI must only build its command tree: the console
+# scripts import it before Click can answer a shell completion request, which
+# the completion scripts make on every keystroke. Every command imports what
+# it runs when it is invoked. Each of these costs tens of milliseconds to
+# import and is the sign that a command implementation is imported too early.
+HEAVY_MODULES = ("pydantic", "requests", "Crypto", "mvt.common.module")
+
+
+@pytest.mark.parametrize("cli_module", ("mvt.ios.cli", "mvt.android.cli"))
+def test_importing_a_cli_does_not_import_the_module_machinery(cli_module, tmp_path):
+    result = run_isolated_python(
+        "import sys\n"
+        f"import {cli_module}\n"
+        f"print(','.join(name for name in {HEAVY_MODULES!r} if name in sys.modules))\n",
+        home=tmp_path / "home",
+    )
+
+    assert result.returncode == 0, result.stderr
+    assert result.stdout.strip() == "", f"{cli_module} imported {result.stdout.strip()}"