swift-package-manager

[DependencyResolver] Start work on a VersionAssignment struct.

 - This is a value type container for a set of package and version assignments,
   along with APIs for querying the induced state of those assignments.

 - It is intended to eventually be backed by some intelligient persistent data
   structure so that the dependency resolution algorithm can rely on being able
   to use this to retain part of the constraint solver state with efficient
   sharing and computation of the constraint query results. Currently none of
   that is present and it is very inefficient.