This creates loading issues on modules depending on storage but not on
the pytest plugin storage exposes. Somehow the loading is eager and not lazy.
Instead we'll explicitely define to modules depending on the pytest plugins in
their root conftest:
```
pytest_plugin = "swh.storage.pytest_plugin"
```
(and adding their deps as swh.storage[testing] if not already and drop the
eventual duplication introduced in tox.ini for the new apparent wrong reasons ;)
[1]
https://docs.pytest.org/en/stable/writing_plugins.html#setuptools-entry-points
Related to T2484