Reimplement DepSet as a wrapper around a generic implementation

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