Serialize workspaceName.

--
MOS_MIGRATED_REVID=103547846
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 39a3de5..b3f75d9 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
@@ -910,7 +910,8 @@
     /**
      * Uses the workspace name from {@code //external} to set this package's workspace name.
      */
-    Builder setWorkspaceName(String workspaceName) {
+    @VisibleForTesting
+    public Builder setWorkspaceName(String workspaceName) {
       pkg.workspaceName = workspaceName;
       return this;
     }