Extract ReverseDepsUtil interface so that InMemoryNodeEntry can be partially isolated from implementation details.

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MOS_MIGRATED_REVID=108523104

src/main/java/com/google/devtools/build/skyframe/BuildingState.java

diff --git a/src/main/java/com/google/devtools/build/skyframe/BuildingState.java b/src/main/java/com/google/devtools/build/skyframe/BuildingState.java
index d0d2573..ad16c38 100644
--- a/src/main/java/com/google/devtools/build/skyframe/BuildingState.java
+++ b/src/main/java/com/google/devtools/build/skyframe/BuildingState.java
@@ -123,7 +123,7 @@
   private boolean reverseDepIsSingleObject = false;
 
   private static final ReverseDepsUtil<BuildingState> REVERSE_DEPS_UTIL =
-      new ReverseDepsUtil<BuildingState>() {
+      new ReverseDepsUtilImpl<BuildingState>() {
         @Override
         void setReverseDepsObject(BuildingState container, Object object) {
           container.reverseDepsToSignal = object;