provenance: Add ComputeDirectoryFrontier
pass 3 out of 3 to implement https://gitlab.softwareheritage.org/swh/devel/swh-graph/-/issues/4782
staging instance, all data can be removed at all time
pass 3 out of 3 to implement https://gitlab.softwareheritage.org/swh/devel/swh-graph/-/issues/4782