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.

docs/development/plugin_configuration.md

@@ -24,8 +24,7 @@ configuration:
 
 The exact parent folder follows the platform convention used for MVT's
 `config.yaml` (for example `~/Library/Application Support/mvt` on macOS). Use
-`mvt.common.plugin_config.plugin_config_path()` instead of building the path by
-hand.
+`plugin_config_path()` from `mvt.plugin` instead of building the path by hand.
 
 Plugin names must be lowercase and may only contain letters, digits and dashes,
 matching the `mvt-plugin-<name>` package naming convention. MVT creates the
@@ -38,9 +37,8 @@ leaves a partially written settings file behind.
 
 Everything else a plugin keeps on disk, such as a cache, a downloaded artifact
 or synchronization state, belongs in the folder returned by the `data_folder()`
-class method of the plugin's settings class, or by
-`mvt.common.plugin_config.plugin_data_folder()` called with the plugin name if
-the plugin has no settings class:
+class method of the plugin's settings class, or by `plugin_data_folder()` from
+`mvt.plugin`, called with the plugin name if the plugin has no settings class:
 
 ```
 ~/.local/share/mvt/plugin-data/<plugin name>/                # Linux
@@ -76,7 +74,7 @@ defaults:
 ```python
 from typing import Optional
 
-from mvt.common.plugin_config import MVTPluginSettings
+from mvt.plugin import MVTPluginSettings
 
 
 class ExamplePluginSettings(MVTPluginSettings):
@@ -94,12 +92,15 @@ from datetime import datetime, timezone
 
 import click
 
+from mvt.plugin import plugin_env_prefix
+
 
 def sync():
     settings = ExamplePluginSettings.load()
     if not settings.API_KEY:
+        prefix = plugin_env_prefix(settings.plugin_name)
         raise click.ClickException(
-            "No API key configured. Set MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY or "
+            f"No API key configured. Set {prefix}API_KEY or "
             "run 'example-plugin configure'."
         )
 
@@ -107,6 +108,9 @@ def sync():
     settings.save()
 ```
 
+The message builds the variable name with `plugin_env_prefix()`, see
+[Environment Variables](#environment-variables).
+
 A missing settings file is not an error: the plugin then runs on the field
 defaults and on whatever the environment provides. `save()` only persists the
 values that differ from the defaults, and it never touches MVT's `config.yaml`.
@@ -129,6 +133,10 @@ export MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY=...
 export MVT_PLUGIN_EXAMPLE_PLUGIN_MAX_RESULTS=50
 ```
 
+Do not repeat the plugin name in a field name: a plugin named `example-scanner`
+with a `SCANNER_API_KEY` field asks the user for
+`MVT_PLUGIN_EXAMPLE_SCANNER_SCANNER_API_KEY`. Name the field `API_KEY`.
+
 Settings resolve in this order, from highest to lowest priority:
 
 1. Arguments passed to the settings class directly, such as