Move VertexIdToIndexMap into impl

graphx/src/main/scala/org/apache/spark/graphx/impl/package.scala

+package org.apache.spark.graphx
+
+import org.apache.spark.util.collection.OpenHashSet
+
+package object impl {
+  type VertexIdToIndexMap = OpenHashSet[VertexID]
+}

graphx/src/main/scala/org/apache/spark/graphx/package.scala

@@ -2,9 +2,7 @@ package org.apache.spark
 
 import org.apache.spark.util.collection.OpenHashSet
 
-
 package object graphx {
-
   type VertexID = Long
 
   // TODO: Consider using Char.
@@ -12,11 +10,6 @@ package object graphx {
 
   type VertexSet = OpenHashSet[VertexID]
 
-  //  type VertexIdToIndexMap = it.unimi.dsi.fastutil.longs.Long2IntOpenHashMap
-  type VertexIdToIndexMap = OpenHashSet[VertexID]
-
-  /**
-   * Return the default null-like value for a data type T.
-   */
+  /** * Returns the default null-like value for a data type T. */
   def nullValue[T] = null.asInstanceOf[T]
 }