Make Repository private[sbt]

This trait may not even survive until 1.4.0. It should definitely not be
public. I got a little overexcited about programming with higher kinded
types when I added it.

main/src/main/scala/sbt/internal/Repository.scala

@@ -33,7 +33,7 @@ import scala.collection.JavaConverters._
  * @tparam K the key type
  * @tparam V the value type
  */
-trait Repository[M[_], K, V] extends AutoCloseable {
+private[sbt] trait Repository[M[_], K, V] extends AutoCloseable {
   def get(key: K): M[V]
   override def close(): Unit = {}
 }