Migrate SkylarkBuiltin annotations to SkylarkSignature instead.

--
MOS_MIGRATED_REVID=91603959
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java b/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java
index 79f2efa..f32dcdf 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java
@@ -14,6 +14,7 @@
 
 package com.google.devtools.build.lib.syntax;
 
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Joiner;
 import com.google.common.cache.CacheBuilder;
 import com.google.common.cache.CacheLoader;
@@ -458,7 +459,8 @@
     }
   }
 
-  static boolean isNamespace(Class<?> classObject) {
+  @VisibleForTesting
+  public static boolean isNamespace(Class<?> classObject) {
     return classObject.isAnnotationPresent(SkylarkModule.class)
         && classObject.getAnnotation(SkylarkModule.class).namespace();
   }