[Source] Fix issue when git's `url.<base>.insteadOf` option is used.

Fixes https://github.com/CocoaPods/CocoaPods/issues/2724,
https://github.com/CocoaPods/CocoaPods/issues/2696, and
https://github.com/CocoaPods/CocoaPods/issues/2625.
[Dependency] Add a cached prerelease? accessor
[Set] Remove unused resolver-related attributes/method
[Specification] Create subspec dependencies with a version requirement of self.version

@kylef this is one performance optimization for the resolver
[Set] Use #join instead of *
[Specification] Add raise_if_missing parameter to subspec_by_name
[CHANGELOG] Add entry for removal of Specification::Set attributes related to dependency resolution