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" ]

The following fix remains with the current other modules [4] 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

• [4] as an example of adaptations D3479 (mercurial loader)

Related to T2484

Test Plan

tox

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Migrated from D3475 (view on Phabricator)