Pre-filter version caches in sm adapter according to root manifest

[sdboyer]

trafficstars

Similar to #3; the SourceManager should receive the root project's manifest from the solver. As it's the only manifest we know will apply, it could be used to preemptively filter the version caches to contain only versions acceptable by those constraints, eliminating the need to redo those computations each and every time a version queue is created for one of those deps.

[sdboyer]

This should be a responsibility of #14

[fabulous-gopher]

This issue was moved to golang/dep#452