stubs for plugin not found when plugin on PYTHONPATH

[jaraco]

Bug Report

In Shoobx/mypy-zope#25, I describe an issue where installing the plugin into a path on PYTHONPATH (but not in site-packages) causes the stubs not to be present, causing failures on a simple check that would otherwise pass. This behavior means that any libraries with stubs cannot be used with tools like pip-run.

To Reproduce

1. Install pip-run. Then:

$ cat > mypy.ini
[mypy]
plugins=mypy_zope:plugin
$ cat > interface.py
from zope.interface.interface import Interface

   
class X(Interface):
    pass
$ pip-run -q --use-pep517 mypy-zope -- -m mypy interface.py
interface.py:1: error: Cannot find implementation or library stub for module named 'zope.interface.interface'
interface.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

See the downstream report for a repro not using pip-run.

Expected Behavior

A plugin should be able to expose stubs by being installed. Is there something the plugin could be doing to avoid this undesirable behavior?

Actual Behavior

Stubs are missing leading to spurious errors.

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: see above
• Mypy configuration options from mypy.ini (and other config files): see above
• Python version used: 3.9.0
• Operating system and version: macOS 10.15

[isac322]

Same on Linux too!

[emmatyping]

Hi, this seems to be a duplicate of #5701 so closing in favor of that issue.