Incorrect handling of `import from` namespace package that spans site-packages and the source tree

Bug Report

The following command:

mypy --explicit-package-bases ns_package/sub_ns_package1/test.py

Resulted in

ns_package/sub_ns_package1/test.py:1: error: Module "ns_package" has no attribute "sub_ns_package2"  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

With the following setup:

Source code tree:

ns_package
    sub_ns_package1
        test.py

(note that there is no __init__.py)

site-packages tree (partial):

ns_package
    sub_ns_package2
        module.py    

(same, no __init__.py)

test.py contents:

from ns_package import sub_ns_package2

While scratching my head I also found the following:

• import sub_ns_package.ns_package2 works.
• it also works if the above import appears together with from ns_package import sub_ns_package2

To Reproduce
See the above.

Expected Behavior

mypy should finish happily in the above example.

Actual Behavior

ns_package/sub_ns_package1/test.py:1: error: Module "ns_package" has no attribute "sub_ns_package2"  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.0.1
• Mypy command-line flags: --explicit-package-bases (tried with or without it) ; --namespace-packages (tried with or without it)
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.8.16

Potential Fix
I have very little knowledge about the codebase, but the following patch would make the problem go away. I'm not sure it's the right fix though.

https://github.com/python/mypy/commit/e69bbd2c82e60640171dd7e2653c010976ad4239

Reproduisez le rapport avec l’arborescence namespace-package indiquée et mypy --explicit-package-bases ns_package/sub_ns_package1/test.py ; inspectez ensuite le commit référencé e69bbd2c82e60640171dd7e2653c010976ad4239. Ajoutez une couverture de régression pour le cas d’un source-tree/site-packages réparti, et vérifiez que la commande se termine sans l’erreur attr-defined.