Make default refs symlinks to detdesc refs

this is similar to what was done in Moore!2082 (merged)

As it stands, we have two copies of the references, the detdesc and the default (dd4hep) ones. Most of the dd4hep tests are failing for more fundamental reasons so we can't update the references there and follow developments (and updates to the detdesc refs).

With this MR, for the time being, we avoid forking the references by having the default (dd4hep) be symlinks to the detdesc ones.