Reorganizes UIKit project a bit for proper target membership

UIGestureRecognizer+Spec is moved to UIKit+PivotalSpecHelperStubs, since it is a stub
PCKMethodRedirector is moved back to the PivotalSpecHelperStubs library, since it supports the stubs
Helpers are renamed to Support
Extensions are renamed to Helpers
Podspec is updated accordingly

[#83510178, #67129146, #67151516]
Fixes UIActivityViewController+Spec to not override its load method, and use PCKMethodRedirector

This change is also needed for CocoaPods users, since CocoaPods installations would have complained about the undeclared selector