core.loader: Migrate away from SWHConfig mixin
Impacts:
- swh-loader-svn!47 (closed): swh.loader.svn
- swh-loader-mercurial!32 (closed): swh.loader.mercurial
- swh-loader-git!51 (closed): swh.loader.git
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 D4124 (view on Phabricator)