Add the mvt.plugin import surface

mvt.plugin re-exports the names a plugin needs from MVT under one import
path. It holds the module base classes and Command, the alert and result
types, the database errors a module raises, the timestamp converters, the
plugin settings API, MVT's settings, get_plugin_logger() and MVT_VERSION.

The names it exports are kept working on a best-effort basis. Changes to
them are announced in the release notes. Anything else in mvt can still be
imported, and may change between releases without notice.

get_plugin_logger(__name__) returns a logger under mvt.ext for plugin code
outside a module class. Its records then reach the console and the
command.log file of a run. A file loaded with --load-module or
--load-command is named after the file.

tests/common/test_plugin.py

@@ -0,0 +1,34 @@
+# 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 mvt.plugin
+from mvt.android.modules.backup.base import BackupModule
+from mvt.common.config import settings
+
+from ..plugin_fixtures import run_isolated_python
+
+
+def test_the_exported_names_are_the_public_names():
+    public = {name for name in vars(mvt.plugin) if not name.startswith("_")}
+
+    assert public == set(mvt.plugin.__all__)
+    assert mvt.plugin.settings is settings
+    assert mvt.plugin.AndroidBackupModule is BackupModule
+
+
+def test_the_surface_imports_before_anything_else_of_mvt(tmp_path):
+    # A plugin can import the surface as its first import of MVT. The
+    # subprocess gets a temporary home because importing MVT writes its
+    # configuration file.
+    result = run_isolated_python(
+        "from mvt.plugin import IOSExtraction, MVT_VERSION, settings\n"
+        "assert MVT_VERSION\n"
+        "assert settings.NETWORK_TIMEOUT > 0\n"
+        "assert IOSExtraction.__name__ == 'IOSExtraction'\n",
+        home=tmp_path,
+    )
+
+    assert result.returncode == 0, result.stderr
+    assert result.stderr == ""