Defining the pytest-plugin though the pytest-plugin [1] makes it loaded by default.
This creates loading issues on modules depending on storage but not on
the pytest plugin storage exposes. It was explained in the doc and I did not realize [2]
Instead we'll explicitely define to modules depending on the pytest plugins in
their root conftest [3]:
```
pytest_plugins = [ "swh.storage.pytest_plugin" ]
```
The following fix remains with the current other modules by:
- adding their deps as `swh.storage[testing]` if not already.
- drop the eventual duplication introduced in tox.ini for the now apparent wrong reasons ;)
- remove the workaround to avoid loading stuff from storage where not needed (swh-web for one)
[1] https://docs.pytest.org/en/stable/writing_plugins.html#setuptools-entry-points
[2] https://docs.pytest.org/en/stable/writing_plugins.html#plugin-discovery-order-at-tool-startup
[3] https://docs.pytest.org/en/stable/writing_plugins.html#requiring-loading-plugins-in-a-test-module-or-conftest-file
Related to T2484