* For now, remove ability to set things like link_with and platform through the target options.
* Resolve all dependencies together and write out all dependencies to Podfile.lock together.
* Add integration spec with multiple platforms.
* 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.
