setup: Do no expose the pytest-plugin any longer

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" ]

[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