Give Sources and SourceProviders a url attribute

This is needed for https://github.com/CocoaPods/CocoaPods/pull/2513.
Maybe we should use ` # rubocop:disable Style/SpecialGlobalVars` for the relevant [line](https://github.com/CocoaPods/Core/pull/174/files#diff-86e9b8098bc786d75ce023c32a8369a4R33) instead. This would allow the check to stay in place or the rest of the code which is preferable.