TarballDirectoryLoader: Ingest tarball like package loaders do
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This now matches the tarball loader behavior (top-level directory included [1]). This also matches what's expected by the guix dataset.
As the nix hashes computed are done from the first directory included in the tarball though, we must also provide that directory. That way, the hashes checks done during ingestion can match appropriately. That was the initial implementation.
In terms of data, as this will change the visit snapshot and the extid mappings, the core loaders (NodeLoader, ...) now declares an extid_version bumped to 1 (it was 0 by default). Which means that all extid mappings will be recomputed.