Ignore CWD for explicit package bases

Feature

Add an option (name TBD) to ignore the current directory when looking for namespace package bases.

As far as I can tell, the behavior would have to be modified here (optionally leaving out [os.getcwd()]: https://github.com/python/mypy/blob/695ea3017fee084c9d2ec17d9b28f8af905e3b63/mypy/find_sources.py#L87

Pitch

When type-checking a repository with src/ structure, there can be conflicts between installed Python modules, and directories which are located at the root of the repository (assuming that is the CWD for running mypy).
I stumbled upon this by having a directory called docker/, while also having https://pypi.org/project/docker/ installed.

As far as I could tell, there's currently no satisfactory way for mypy to accept this, besides renaming the directory.

Happy to take a stab at implementing this, if you feel it's worth adding.

Related discussion: #8584

Start in mypy/find_sources.py around line 87, where the current directory is included when finding namespace package bases. Read related discussion #8584 before deciding how the new option should be named and scoped. Done means an explicit package-base configuration can ignore the current directory without losing the intended source discovery behavior.