Added ProjectQueryPlatformSource.
Slighlty adjusted the specification of GradleProjectPlatformQuery.
Added ProjectPlatformSource including some related refactoring.
ProjectPlatform sources now do not return null, instead they return the default platform.
Added a default implementation for GradleProjectPlatformQuery.
Relying on GradleProjectPlatformQuery. Also, ProjectPlatform is now an interface.