(Bug 49405) Dont rely on dp.src while serializing tpls/exts/params

* Even though the serializer had the ability to serialize new
  transclusions, extension uses, or params, the serializer handler
  fetching code was still checking for the presence of original
  source for these uses which prevented new <ref> tags from being
  serialized properly.

* No change in parser test results.

Change-Id: I797b2bd87da10cd44c5eecd7b5c814777ea1a05e