Add SvnDirectoryLoader in charge of ingesting svn tree

This overrides the default core class DirectoryLoader's fetch_data method to export an svn tree at a specific commit or tag. The remaining behavior is the same as the DirectoryLoader. Checks the checksums of the tree is ok, then ingest the DAG objects into the archives (including the NAR as extid).

It's wip as this needs more tests. I'm in the middle of rewriting the "base" DirectoryLoader to something more suitable for this MR (to avoid duplication).

@vlorentz might be interested to have a look nonetheless ;)

Refs. swh/meta#4979 (closed)