Enable cross referencing for generated sources (#1057)

This change should allow generated classes like AutoValue or Dagger
classes to be cross-referencable on cs.nomulus.foo

See b/184284124 for context.

vnames.json

@@ -1,6 +1,6 @@
 [
   {
-    "pattern": "(build/[^/]+)/(.*)",
+    "pattern": "([^/]+/build/[^/]+)/(.*)",
     "vname": {
       "corpus": "github.com/google/nomulus",
       "path": "@2@",