mypy_cache conflicts with namespace packages

[brandonchinn178]

Our project uses --explicit-package-bases and the default --namespace-packages. It seems like with these options, mypy will use a directory specified in MYPYPATH over a directory relative to CWD. rules_mypy creates .mypy_cache/ at bazel-out/*-fastbuild-*/bin/path/to/dir/dir.mypy_cache/ AND adds bazel-out/*-fastbuild-*/bin to MYPYPATH, causing it to find that directory instead of the path/to/dir/foo.py file in the standard file system.

I see generated_dirs is used to construct generated_import_dirs

rules_mypy/mypy/private/mypy.bzl

Lines 140 to 143 in da53ac1

	generated_imports_dirs = []
	for generated_dir in generated_dirs.keys():
	for import_ in imports_dirs.keys():
	generated_imports_dirs.append("{}/{}".format(generated_dir, import_))

Is there a reason why generated_dirs is added to MYPYPATH in addition to generated_import_dirs?

Related: python/mypy#19129