Use WorkspaceNameValue.key() to seed RBuildFile traversals under WORKSPACE file modifications.

Move some RBuildFilesVisitor-specific logic out to that file for better organization and to remove some clutter from SkyQueryEnvironment

PiperOrigin-RevId: 296281889
diff --git a/src/main/java/com/google/devtools/build/lib/query2/ParallelSkyQueryUtils.java b/src/main/java/com/google/devtools/build/lib/query2/ParallelSkyQueryUtils.java
index 9e81dbd..1034dc9 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/ParallelSkyQueryUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/ParallelSkyQueryUtils.java
@@ -154,7 +154,7 @@
             /*resultUniquifier=*/ env.createSkyKeyUniquifier(),
             context,
             callback);
-    visitor.visitAndWaitForCompletion(env.getSkyKeysForFileFragments(fileIdentifiers));
+    visitor.visitFileIdentifiersAndWaitForCompletion(env.graph, fileIdentifiers);
   }
 
   static QueryTaskFuture<Void> getDepsUnboundedParallel(