Add the mvt.plugin import surface (#901)

* 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.

* Document how to write MVT plugins

The custom modules page now leads with plugin packages. Loading module
files with --load-module and MVT_CUSTOM_MODULES moves to a section on
developing a module locally.

A new "Writing a module" section shows a module which subclasses
IOSExtraction. It lists each base class, the command pair it serves and the
helpers it provides. "Depending on a built-in module" says to import a
built-in class from its family package.

"Importing from MVT" says what mvt.plugin exports and what importing from
it means. The custom commands page shows a Command subclass which lists its
own modules. The sysdiagnose and plugin configuration pages import from
mvt.plugin.

tests/plugin_fixtures.py

@@ -73,6 +73,7 @@ def run_isolated_python(
     }
     isolated_environment["HOME"] = str(home)
     isolated_environment["XDG_CONFIG_HOME"] = str(home / "config")
+    isolated_environment["XDG_DATA_HOME"] = str(home / "data")
     if site_path is not None:
         isolated_environment["PYTHONPATH"] = str(site_path)
     isolated_environment.update(environment)