Only build framework schemes

Fixes #124, #143. Tested with the unit test fixture and also the `Cartfile` described in the original issue.

/cc @0xced
You’re stating the type of the closure explicitly here, but in general I would avoid explicit returns because a) IMO they’re chaff, and b) in some cases return type inferencing will fail with `return` but succeed without; IIRC there’s some detail about type inferencing not being able to cross statement boundaries.