TarballDirectoryLoader: Adapt tarball ingestions

This roughly matches how the tarball loaders do ingest the tarball. They
ingest the top-level directory [1]. As nixpkgs computes their hashes from the
first directory though, we also provide that directory so they can match their
extid accordingly.

In terms of data, this will change the visit snapshot and the extid
mappings. As a result, the core loaders (NodeLoader, ...) have now an
extid_version bumped to 1 (it was 0 by default).

[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