Expose the proper plugin in the published mypy version

Feature
The misc/proper_plugin.py is currently not packaged with mypy. Can we expose it as part of mypy itself?

Pitch
I think it would be wise to have a single source of truth for the proper_plugin.py. As the mypy project already uses it to validate internal plugins like the dataclasses, it should stay up to date. Some projects copy the file to their project, but that can cause confusing from not being up to date (i.e. https://github.com/python/mypy/issues/8047#issuecomment-560124874). I think externally developed plugins could benefit from the plugin. So perhaps exposing it as mypy.misc.proper_plugin?

misc/proper_plugin.py と、要求されている mypy.misc.proper_plugin のインポートパスから始めます。公開されている mypy パッケージが現在どのようにモジュールを含めているかを確認し、必要なパッケージングの変更を判断します。プロジェクトがコピーしなくても、公開されている mypy パッケージを通じて proper プラグインを利用できれば完了です。