Inject the default runfiles prefix from the rule class provider.

--
MOS_MIGRATED_REVID=102341264
diff --git a/src/main/java/com/google/devtools/build/lib/packages/RuleClassProvider.java b/src/main/java/com/google/devtools/build/lib/packages/RuleClassProvider.java
index d153fa1..958d88a 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/RuleClassProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/RuleClassProvider.java
@@ -32,6 +32,11 @@
   PathFragment getPreludePath();
 
   /**
+   * The default runfiles prefix (may be overwritten by the WORKSPACE file).
+   */
+  String getRunfilesPrefix();
+
+  /**
    * Returns a map from rule names to rule class objects.
    */
   Map<String, RuleClass> getRuleClassMap();