fixes mitogen-hq#891

(cherry picked from commit 1a84184)

Co-authored-by: Stefano Rivera <[email protected]>

When the requested module (e.g. ansible.module_utils.distro)
- is provided by another module *e.g. distro)
- that itself was a package (e.g. distro 1.7.0)

At runtime
- ansible/module_utils/distro/__init__.py executes
- if https://pypi.org/project/distro/ is present, it's loaded as
ansible.module_utils.distro
- otherwise ansible/module_utils/distro/_distro.py is loaded

ParentEnumerationMethod would wrongly use whatever was in
sys.modules['ansible.module_utils.distro]. Instead we should ascend to
the first parent that has fullname == sys.modules[fullname].__name__.
Then descend to the appropriate .py file on disk.

This bug didn't show up before because until distro 1.7.0 (Feb 2022) the
top-level distro module was a module (distro.py) not a package
(distro/__init__.py)

fixes mitogen-hq#906