This provides a new Directory loader implementation for svn trees. Only the
fetch_artifact implementation is required (and provided) 
This reuses the loader svn's client to export an svn tree at a specific commit or tag. The remaining behavior is the same as the BaseDirectoryLoader. Checks the checksums of the tree is ok, then ingest the DAG objects into the archives (including the NAR as extid).
It now depends on a fix in the loader-core too  (so build will fail as this needs a bump in the dependency).
 Depends on swh-loader-core!469 (merged)
 Depends on swh-loader-core!471 (merged)
Refs. swh/meta#4979 (closed)