from_disk: Update loader classes implementation

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.