Error when running on code inside venv containing `typing-extensions`

Bug Report

When running mypy on code installed in a virtualenv environment, mypy will fail outright if the environment contains typing-extensions.

To Reproduce

See repo containing a minimal example. The steps below describe the general idea:

1. Create a fresh virtual environment.
2. Create a minimal library with a setup.py that lists typing-extensions in install_requires.
3. Install the library to the virtual environment.
4. Run mypy on a file inside the environment that is part of the library (mypy venv/lib/python3.Y/site-packages/my_project/hello.py).

Expected Behavior

mypy would report errors in hello.py, if any, or exit with 0 if none.

Actual Behavior

mypy doesn't check hello.py, complaining that it found a typing_extensions.py in the environment:

mypy: "project-venv/lib/python3.6/site-packages/typing_extensions.py" shadows library module "typing_extensions"
note: A user-defined top-level module with name "typing_extensions" is not supported

Your Environment

• Mypy version used: 0.910.
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: multiple (3.6.8, 3.8.8).
• Operating system and version: CentOS 7

Inizia con l'esempio minimo nel repository mypy-typing-extensions-issue collegato e riproduci il comando con un pacchetto installato in un ambiente virtuale. Traccia l'entry point della risoluzione dei moduli di mypy per l'errore di shadowing segnalato, quindi verifica che la hello.py di destinazione venga controllata come previsto senza l'errore.