Implement package type/api compatibility checking

[sdboyer]

trafficstars

We need to add a satisfiability condition that ensures type compatibility between packages at a given version. Nontrivial, but also shouldn't be crazy-difficult. Once we add this, though, we'll really need to start persisting the SourceMgr caches to disk.

[fabulous-gopher]

This issue was moved to golang/dep#438