swift-package-manager

[PackageGraph] Change loader to take pre-loaded manifests.

 - I think I have realized it is more appropriate for the loader to be decouple
   from the thing responsible for loading what is effectively the manifest
   graph.

 - This leaves PackageGraphLoader with a relatively thin set of
   responsibilities, but I think it is appropriate to still have it to decouple
   the _loading_ process from the fully formed graph data structure.