Represent "virtual" segments like route groups (#82422)
Small update to #82383. Instead of using a special symbol to represent
the "virtual" segments that get inserted into the loader tree for
parallel routes, we can represent them like route groups, since they
serve the same purpose. That will allow us to remove a special case in
the param parsing logic, since we have to skip groups regardless; both
will be skipped by the same check.
