[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 
Gotcha, that makes total sense. I just want it clarified that it’s only the case in certain circumstances and what those are ;)