Fix monkeypatch resolution of cached submodules

[SharifWaqas]

Fix string-based monkeypatch.setattr and delattr for cached submodules whose parent-package attribute is missing. Use the module returned by importlib. Regression tests cover both operations and undo.

Validation: 4,688 tests passed with no failures. Both regressions fail before the fix. Pre-commit checks passed.

Developed with OpenAI Codex assistance. The temporary changelog/0.bugfix.rst will be renamed to this PR number.

[0xamlab]

Verified the fix locally. The new regression tests in testing/test_monkeypatch.py pass on this branch (46 passed, 2 skipped in testing/test_monkeypatch.py), and I reproduced the failure on main: mp.setattr("cached_target_package.child.value", 2) raises AttributeError: 'module' object at cached_target_package.child has no attribute 'child' when the submodule is cached but no longer exposed on its parent. The mechanism is the capture of the module returned by importlib.import_module at src/_pytest/monkeypatch.py:82, instead of falling through to annotated_getattr on the parent package. Clean change, good test coverage for setattr, delattr, and undo.