Fix caching of PEP 561 namespace packages

[hauntsaninja]

Fixes #13085. Hopefully more robust than previous fixes along these lines.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

ignite (https://github.com/pytorch/ignite)
- ignite/contrib/engines/common.py:6: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+ ignite/contrib/engines/common.py:10: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

poetry (https://github.com/python-poetry/poetry)
- src/poetry/config/config.py:13: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+ src/poetry/config/config.py:14: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

arviz (https://github.com/arviz-devs/arviz)
+ arviz/plots/backends/matplotlib/traceplot.py:5: error: Cannot find implementation or library stub for module named "matplotlib"
- arviz/plots/backends/matplotlib/traceplot.py:6: error: Cannot find implementation or library stub for module named "matplotlib"
+ arviz/plots/backends/matplotlib/kdeplot.py:3: error: Cannot find implementation or library stub for module named "matplotlib"
- arviz/plots/backends/matplotlib/kdeplot.py:5: error: Cannot find implementation or library stub for module named "matplotlib"
+ arviz/plots/backends/matplotlib/elpdplot.py:4: error: Cannot find implementation or library stub for module named "matplotlib"
- arviz/plots/backends/matplotlib/elpdplot.py:5: error: Cannot find implementation or library stub for module named "matplotlib"
+ arviz/plots/backends/bokeh/khatplot.py:4: error: Cannot find implementation or library stub for module named "matplotlib"
- arviz/plots/backends/bokeh/khatplot.py:5: error: Cannot find implementation or library stub for module named "matplotlib"

vision (https://github.com/pytorch/vision)
- torchvision/__init__.py:4: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+ torchvision/__init__.py:15: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

jax (https://github.com/google/jax)
- jax/_src/lib/__init__.py:38: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+ jax/_src/lib/__init__.py:48: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

pyinstrument (https://github.com/joerick/pyinstrument)
+ pyinstrument/vendor/appdirs.py:533: error: Cannot find implementation or library stub for module named "com.sun"
- pyinstrument/vendor/appdirs.py:562: error: Cannot find implementation or library stub for module named "com.sun"
- pyinstrument/magic/magic.py:6: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+ pyinstrument/magic/magic.py:7: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

bokeh (https://github.com/bokeh/bokeh)
- bokeh/core/serialization.py:49:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+ bokeh/core/serialization.py:53:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

[JukkaL]

Thanks for the fix! The logic at least looks more straightforward now, so hopefully it fixes the issue for good.