Fix pytest collection during test_host partial run

Sometimes pytest errors out with an `ImportError` during its tests
collection stage when a Python package/directory containing the test
module does not have an `__init__.py` in it. This is being observed
under Python 3.9 and higher.

The patch provides a workaround for this problem but does not address
the root cause which is currently unknown.

Ref:
ansible#78585 (comment)
(cherry picked from commit beb70da)