packaging: Split swh.core and test dependencies properly

It's a mixed issue of python module (pip declaration deps) and python debian packages.

swh.core (thus python3-swh.core) now pulls transitively the python3-postgresql server. This means, it installs, within debian stable, the postgresql server wherever a package installs core (so pretty much every machine...).

Rework module and packaging so it's only a test dependency when necessary should fix that issue (well that and cleaning up machines...)