Refine the implementation of the directory_ls function
- use the graph to retrieve not the full directory, but only a few layers --> set max_depth to 2 or 3 here, to be refined as needed https://gitlab.softwareheritage.org/douardda/swh-provenance/-/blob/test-docker/swh/provenance/archive/swhgraph.py?ref_type=heads#L33
- cache the resulting tree (incrementally, i.e. all the layers seen are in the cache)