setup: Do no expose the pytest-plugin any longer
Defining the pytest-plugin though the pytest-plugin  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 
Instead we'll explicitely define to modules depending on the pytest plugins in
their root conftest :
pytest_plugins = [ "swh.storage.pytest_plugin" ]
Related to T2484