TarballDirectoryLoader: Ingest tarball as package loaders do

This is source of discrepancy for origins with type tarball-directory which do
not have the top-level directory. This also matches how the package loader are
currently ingesting artifacts [1]

In terms of data, this will change the visit snapshot.

In terms of ui browsing, this won't change a thing since the webapp already
has heuristic to naturally skip the unique top-level directory.

[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