Skylark: Expose output_to_genfiles rule attribute

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MOS_MIGRATED_REVID=90616271
diff --git a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
index 3c8172a..d265a44 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
@@ -1508,7 +1508,8 @@
    * individual rule instance, derived from the 'output_to_bindir' attribute;
    * see Rule.hasBinaryOutput().
    */
-  boolean hasBinaryOutput() {
+  @VisibleForTesting
+  public boolean hasBinaryOutput() {
     return binaryOutput;
   }