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)

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Migrated from D4124 (view on Phabricator)