rpc-server: Use directly the package's server module to start server

This stops using the wsgi module indirection. Which caused unneeded complication [1].

I think those modules were to match what we do for deposit and swh-web (django apps). Because django's doc mentions wsgi modules [2] [3]

So, in the end, it's not necessary to follow that convention here. That introduces indirection and unneeded state (which can cause problem).

This will allow for the respective python3 packages to drop that unnecessary module (which is wip on my machine right now).

• [1] Related D2101#48716

• [2] https://docs.djangoproject.com/en/2.2/howto/deployment/wsgi/gunicorn/

• [2] https://docs.djangoproject.com/en/2.2/howto/deployment/wsgi/

Test Plan

staging:

• apply the diff's content
• restart the impacted services
• everything is fine

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Migrated from D2109 (view on Phabricator)