Support loading stubs/ from typeshed

[not-my-profile]

When I create a file foo.py with import toml and execute mypy foo.py --custom-typeshed-dir my-typeshed/ I get:

foo.py:1: error: Library stubs not installed for "toml" (or incompatible with Python 3.9)
foo.py:1: note: Hint: "python3 -m pip install types-toml"
foo.py:1: note: (or run "mypy --install-types" to install all missing stub packages)
foo.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

Although my-typeshed/stubs/ contains toml/.

I find this really inconvenient because I'd like to test my third-party stub typeshed contributions with mypy.

It's worth noting that pyright handles this, pyright foo.py --typeshed-path my-typeshed/ works as expected.

[Akuli]

This works, but is probably not documented anywhere and we should at least document it: MYPYPATH=stubs/toml/ foo.py