Make type checking mypy more straightforward for new contributors

There are a couple pitfalls relating to type checking mypy itself that don't seem to be documented anywhere:

1. Specifying files: Running mypy . and mypy mypy mypyc both result in duplicate module errors that prevent further type checking. The correct way to run mypy is mypy -p mypy -p mypyc, which doesn't appear to be documented anywhere (I only found this by looking at the tox.ini config file).
2. Specifying the config file: Failing to specify mypy_self_check.ini as the config file results in an error in mypy/bogus_type.py and means that people aren't using mypy's stricter type checking options.

I think the solution to 2 would be to rename mypy_self_check.ini to mypy.ini, because self checking mypy is significantly more common than compiling mypy with mypyc, so making mypy_self_check.ini the default config file seems reasonable.

For 1, we should at the very least document the command to run mypy in CONTRIBUTING.md. I think the ideal solution would involve changing the configuration file so that either mypy . or mypy mypy mypyc just works correctly, the latter of which seems to be possible from my experiments by turning on namespace_packages and excluding mypyc/test-data.

Start with CONTRIBUTING.md and tox.ini to compare the documented and configured mypy commands. Inspect mypy_self_check.ini and mypy/bogus_type.py, then verify whether the proposed config naming or namespace-package changes avoid duplicate-module errors while retaining strict self-checking. Done means the contributor workflow documents a reliable command and configuration.