Backtracking for dependency resolver

[MartinNowak]

Both bundler and carge seem to have good experience with backtracking to speed up dependency resolution and print humanly understandable conflicts.
https://github.com/CocoaPods/Molinillo/blob/b97b2f96440925b93150de20709c6403c5ee22f0/ARCHITECTURE.md#backtracking
rust-lang/cargo#5187

[gedaiu]

this sounds like an awesome idea!