swift-package-manager

[Build] Eliminate unused package version metadata feature.

 - This feature is only partially implemented, and isn't actually used by
   anything. Unfortunately, it also conflicts with some of the work I need to do
   on SE-0135 in a way that is non-trivial to fix right now. I'm ripping it out
   for now, we can land it again once this feature has been fully designed and
   implemented.