This is a proof-of-concept for pytest-dev/pytest#8964 (comment).

The idea is to import test packages on the requested test paths if pytest runs with --import-mode=importlib.

$ python setup.py develop
$ pytest --import-mode=importlib tests/

Although tests/test_foo.py features an import from tests/utils.py, we can pytest --import-mode=importlib tests/ without error.