mercurial loader: Refactor / clean up old implementations and rename appropriately the official one

There exists multiple implementations of the mercurial loader. Only one is now the official swh.loader.mercurial.from_disk.HgLoaderFromDisk.

The other implementations helped for backward compat when crafting the new one. Now that's it's landed and used, those should be cleaned up to avoid a high burden maintenance.

The official one should be renamed to swh.loader.mercurial.loader.HgLoader.

That might be a blocking point for [1] so that may have to come before it.

• [1] #3571 (closed)

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Migrated from T3572 (view on Phabricator)