fix debian packages' issues

• swh-web does not build because it depends on swh-indexer's build working (which does not on unstable)
• swh-indexer does not build because it needs universal-ctags on unstable (not packaged there ~> i backported it from buster to our debian.s.o)
• swh-indexer still does not build because it depends on swh-scheduler's build
• swh-scheduler does not build because of an unpackaged package dependency

As a workaround for now, to deploy the current swh-web, swh-indexer, etc..., i'll mark the db tests as such (they are skipped during debian package build). The real solution though would be to package the missing fixture [2].

• [1] https://jenkins.softwareheritage.org/view/Debian%20packages/job/debian/job/packages/job/DSCH/job/gbp-buildpackage/3/console

• [2] https://pypi.org/project/pytest-postgresql/

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Migrated from T1498 (view on Phabricator)