Make the rule in the workspace package reference the correct Package object.

Fixes #1228.

--
MOS_MIGRATED_REVID=122511655
diff --git a/src/main/java/com/google/devtools/build/lib/packages/Package.java b/src/main/java/com/google/devtools/build/lib/packages/Package.java
index 1776ed6..fc2a193 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/Package.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/Package.java
@@ -1190,6 +1190,7 @@
     }
 
     void addRule(Rule rule) throws NameConflictException {
+      Preconditions.checkArgument(rule.getPackage() == pkg);
       checkForConflicts(rule);
       // Now, modify the package:
       for (OutputFile outputFile : rule.getOutputFiles()) {