docker-dev: Use loader-package implems from swh.loader.core modules

This also removes the potential [1] breaking of the different services (referencing the old task names without the module installed)

• [1] I did not see it in the earlier diff (!112 (closed)).

I also did not yet tested it entirely (in-progress, that should not prevent from merging as it stabilizes a bit more already). Now swh-loader image starts.

Related T1389

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Migrated from D2305 (view on Phabricator)