Make Rule know about the name of the workspace it is in.

This is needed for taking the runfiles prefix from the WORKSPACE file instead of hardcoding it.

--
MOS_MIGRATED_REVID=87347883
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
index 0af667a..1bbcd72 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
@@ -159,6 +159,13 @@
   }
 
   /**
+   * Returns the workspace name for the rule.
+   */
+  public String getWorkspaceName() {
+    return rule.getWorkspaceName();
+  }
+
+  /**
    * The configuration conditions that trigger this rule's configurable attributes.
    */
   Set<ConfigMatchingProvider> getConfigConditions() {