pathlib: Fix module path used when using importlib with namespaces

[isra17]

Closes #12044

Will do other steps from the template, but before I would like a confirmation that this fix make sense.

It does fix the issue of assertion rewriting mixed with namespace package on in my environment.

If fixes both the cases where the tests lives in a package with __init__.py and with the config consider_namespace_packages = true. I am not familiar at all with this part of the code (or any to be honest), so I'm unsure what are the consequence, but from my understanding the from_spec(module, path) path parameter should take the module parent path, not the source root.

From the doc:

path will be the value of path from the parent package.

@nicoddemus since you seem the main maintainer of this part.

Thanks!

[isra17]

On a local test:

src/
  p1/foo/p1/
    tests/
      test_p1.py
    __init__.py
    test_p1.py
  p2/foo/p2/
    __init__.py
    test_p2.py

Without the fix:

FAILED src/p1/foo/p1/test_p1.py::test_p1 - AssertionError
FAILED src/p1/foo/p1/tests/test_p1.py::test_p1 - AssertionError
FAILED src/p2/foo/p2/test_p2.py::test_p2 - AssertionError

With the fix:

FAILED src/p1/foo/p1/test_p1.py::test_p1 - assert False is True
 +  where False = p1()
FAILED src/p1/foo/p1/tests/test_p1.py::test_p1 - assert False is True
 +  where False = p1()
FAILED src/p2/foo/p2/test_p2.py::test_p2 - assert False is True
 +  where False = p2()