package.loader: Migrate away from SWHConfig mixin
Impact is already dealt with within that diff (all package loaders are defined in the same repository).
Related to swh-core#1532 (closed) swh-core#1410 swh-core!159 (closed)
Test Plan
tox (failing until swh.core > 0.3.1, including swh-core!159 (closed), is released)
Migrated from D4125 (view on Phabricator)