Add spec linting

[yonaskolb]

This would run the current spec validator (which highlights errors), but also a seperate linting phase that would provide suggestions on how to simplify the spec using all it's features eg. use the deploymentTarget instead of setting IPHONEOS_DEPLOYMENT_TARGET

[yonaskolb]

This could have a fixit mode that automatically applied suggestions

[Lutzifer]

Almost 5 Years later: Is there something like this? I just spent 2 hours debugging, because my dependencies were indented one level too deep...

[yonaskolb]

This doesn't exist yet, but I image it would share some code with what's needed in the migration command https://github.com/yonaskolb/XcodeGen/pull/735