[Set] Sources are used in the order in which they are provided

These changes are needed for https://github.com/CocoaPods/CocoaPods/pull/2595.

The resolver now creates `Set` objects providing the sources that the user defined in the Podfile, in the order that the user specified them in. The `Set` object should therefore not use them alphabetically anymore.

:rainbow: 
`CocoaPods#2556`