Implement depSet as a generic depsets implementation using reflection,
and then make DepSet a type-safe wrapper around it. This will allow
additional wrappers for depsets that work with other types. All of
this can be replaced with generics once Go supports them.
Test: depset_test.go
Change-Id: Id9df17bcc76f6c1545e7eb498f298066cf8a7679
Colin Cross