* No need to pass the project path and podfile separately to UserProjectIntegrator.
* Move Platform#xcodeproj to TargetDefinition#link_with.
* Add multiple-platforms section to changelog.
* Validate platforms for each target definition.
* Validate that there are dependencies in a Podfile.
* Validate that the dependencies in the targets don't conflict. E.g. two different versions of the same pod.
* Move Podfile.lock generator from Installer into its own file.
