mercurial loader: Align mercurial loader with other loaders

Mainly, that means, make it inherit from the loader-core's base loader.

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Migrated from T907 (view on Phabricator)

assigned to @ardumont

marked this issue as related to #329 (closed)

added Mercurial loader priority:Normal labels

mentioned in commit swh-loader-core@80317628

mentioned in commit swh-loader-git@19df6d0d

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closed