collect sub collections.scala

    def collectSubAar(ps: Seq[ProjectRef]): Seq[AarLibrary] = ps flatMap { p =>
      val locals = Project.runTask(libraryProjects in(p, Android), st).collect {
        case (_, Value(value)) => value
      }
      val aarlibs = locals.fold(Seq.empty[AarLibrary])(_ collect {
        case a: AarLibrary => a
      })
      val sub = Project.getProject(p, struct)
      
      if(sub.isEmpty) aarlibs
      else aarlibs ++ sub.fold(Seq.empty[AarLibrary])(s => collectSubAar(s.dependencies.map(_.project)))
    }