[WIP] [Resolver] Add filtering for subspecs included via subspec_dependencies

Depends upon https://github.com/CocoaPods/Core/compare/seg-from-subspec-dependencies.

Closes https://github.com/CocoaPods/CocoaPods/issues/2757. 

\c @alloy @neonichu @kylef 
Can you update this to state it is about implicitly activated subspecs? (i.e. no `default_subspec`.)