Expose referenc*ing* projects on project graph nodes

Some potential consumers of this API have expressed that they would find it very useful to have links back up to referencing projects, as opposed to only referenced projects.

The main reason we punted on this originally was simplicity and because coming up with clear names is incredibly hard (referencing vs referenced, parent vs child, etc).

This is mostly for convenience as API consumers could build a structure of reverse edges themselves by traversing the graph once.