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from_disk: Update loader classes implementation

Antoine Lambertrequested to merge
anlambert/swh-loader-git:update-from-disk-implementation into master

GitLoaderFromDisk and GitLoaderFromArchive implementation can now be simplified by inheriting from GitLoader class and by overriding the fetch_pack_from_origin method to fetch git objects from disk instead of fetching them from a remote URL.

Also remove the deprecation notice for GitLoaderFromDisk as it is likely there will be use cases in a near future for that git loader.

Related to !190 (comment 183093)

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