pytest 8.3.4 --import-mode importlib regression

[alund]

Starting in 8.3.4, running pytest using --import-mode importlib with the following directory structure no longer works.

<root>
    + mypkg
        - pyproject.toml
        + mypkg
            - __init__.py
        + tests
            - test_version.py

test_version.py attempts to import from mypkg and fails:

mypkg/tests/version_test.py:1: in <module>
    from mypkg import version
E   ImportError: cannot import name 'version' from 'mypkg' (unknown location)

pytest.ini contains:

[pytest]
addopts = --import-mode importlib
testpaths =
    mypkg/tests

git bisect says:

± git bisect bad
3d3ec5724c6f76bc07d0631ec8061f26f9ecac4c is the first bad commit
commit 3d3ec5724c6f76bc07d0631ec8061f26f9ecac4c
Author: patchback[bot] <45432694+patchback[bot]@users.noreply.github.com>
Date:   Thu Sep 26 01:26:01 2024 +0000

    Fix KeyError with importlib mode (directories with same name) (#12752) (#12843)

    Directories inside a namespace package with the same name as the namespace package would cause a `KeyError` with `--import-mode=importlib`.

    Fixes #12592

    Co-authored-by: Bruno Oliveira <bruno@pytest.org>
    (cherry picked from commit 6486c3f3a858a0c8043f5c3f7c24297b82a0abe4)

    Co-authored-by: dongfangtianyu <7629022+dongfangtianyu@users.noreply.github.com>

 changelog/12592.bugfix.rst |   1 +
 src/_pytest/pathlib.py     | 131 +++++++++++++++++++++++++++++++++------------
 testing/test_pathlib.py    |  74 ++++++++++++++++++++++++-
 3 files changed, 171 insertions(+), 35 deletions(-)
 create mode 100644 changelog/12592.bugfix.rst 

I'm attaching a minimal project that should reproduce the problem.
reproduce.zip

[vismaytiwari]

Opened #14709 for this. Your bisect was spot-on — #12752 made importlib mode eagerly build the parent packages, so with the mypkg/tests/... layout pytest binds the outer mypkg directory as a namespace package and shadows the real installed mypkg, which is why from mypkg import version comes back with (unknown location).

The fix defers to the real package when one actually lives inside that directory, so from mypkg import version resolves to the installed package again. A test directory named after a stdlib module still gets a namespace package, so the earlier same-name fixes (#12592, #13026) stay intact.

Verified against your reproduce.zip: passes on 8.3.3, failing on 8.3.4 and main, passing with the change.