pytest_plugin: Avoid fixture client to declare optional dependency
Before this commit, this would make swh_storage_backend_config fixture clients require to declare an optional dependency on swh.journal. Otherwise, it would not build [1].
This commit fixes it by dropping this configuration in the main pytest plugin. It keeps the storage tests testing with that journal_writer collaborator though (as initially) by declaring an override which still provides it.
This fixes the debian build [1]
This replaces swh-loader-core!108 (closed) by fixing the problem at the source. swh-loader-core!108 (closed) took the path to change the configuration within the clients. This is not sustainable as this would require other fixture clients to do the same.
Test Plan
tox
Migrated from D3515 (view on Phabricator)