Add mercurial directory loader

This provides the class HgDirectoryLoader with only the fetch_artifact method. This clones a repository at a specific changeset (or tag) and ingests the DAG objects from the resulting directory tree. It also checks the checksums (Nar or standard) if provided.

The Nar checksums are stored in the extid mapping table when provided (when checks are fine).

Refs. swh/meta#4979 (closed)