TarballDirectoryLoader: Ingest tarball like package loaders

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.

[1] https://gitlab.softwareheritage.org/swh/devel/swh-loader-core/-/blob/master/swh/loader/package/loader.py?ref_type=heads#L829-837

Refs. swh/infra/sysadm-environment#5222