directory: Allow to use an origin URL different from the root svn URL
When the SvnExportLoader must export multiple subsets of a subversion tree using the same root svn URL, the same origin URL should not be used in order to disambiguate what is archived.
So allow to specify the root svn URL in an optional svn_url parameter of the loader while a different origin URL is provided as first positional parameter of the loader.
Related to swh/infra/sysadm-environment#5263 (closed).