Some Guix packages correspond to subset exports of a subversion source tree at a given revision, typically the Tex Live ones.
In that case, we must pass an extra parameter to the svn-export loader to specify the sub-paths to export but also use a unique origin URL for each package to archive as otherwise the same one would be used and only a single package would be archived.
Depends on !522 (merged). Depends on swh-loader-svn!247 (merged).
Related to swh/infra/sysadm-environment#5263 (closed).