mercurial.loader: Migrate away from SWHConfig mixin

Related to swh-core#1532 (closed) swh-core#1410 swh-core!159 (closed)

Test Plan

tox

and docker:

time doco exec swh-loader swh loader run mercurial https://forge.extranet.logilab.fr/open-source/pybill                                         INFO:swh.loader.mercurial.Bundle20Loader:Load origin 'https://forge.extranet.logilab.fr/open-source/pybill' with type 'hg'
{'status': 'eventful'}
docker-compose exec swh-loader swh loader run mercurial   0.50s user 0.44s system 4% cpu 19.792 total

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Migrated from D4127 (view on Phabricator)