nixguix: Handle creation of svn-export visit types on svn sub-trees

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.

Related to swh/infra/sysadm-environment#5263.