"Bundled" plugins in mypy live here  https://github.com/python/mypy/tree/master/mypy/plugins (mostly one plugin per file, there are some examples for existing stdlib plugins like ctypes, enum, dataclasses)

The plugin hooks should be then added to `DefaultPlugin` in `default.py`.

The tests can probably go into https://github.com/python/mypy/blob/master/test-data/unit/check-default-plugin.test (not really important where to put them)

If you will make a PR, then someone will likely guide you through if there are issues. Can't guarantee a quick response however.

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Ivan