setup: Do no expose the pytest-plugin, let modules require it

Defining the pytest-plugin though the pytest-plugin [1] makes it loaded by default. This creates loading issues on modules depending on scheduler but not on the pytest plugin scheduler exposes as explained in the doc [2]

Instead we'll explicitely define to modules depending on the pytest plugins in their root conftest [3]:

pytest_plugins = [ "swh.scheduler.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 swh-storage!945 (closed) Related to T2484

Note: that will fix the debian build [4]

• [4] https://jenkins.softwareheritage.org/view/Debian%20packages/job/debian/job/packages/job/DLDBASE/job/gbp-buildpackage/150/

Test Plan

tox

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Migrated from D3492 (view on Phabricator)