Use a factory method in ASTFileLookupValue to enforce that the NO_FILE constant is correctly used.

--
MOS_MIGRATED_REVID=87008436
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/PackageFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/PackageFunction.java
index c106081..b318f3e 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/PackageFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/PackageFunction.java
@@ -436,7 +436,7 @@
     if (astLookupValue == null) {
       return null;
     }
-    List<Statement> preludeStatements = astLookupValue == ASTFileLookupValue.NO_FILE
+    List<Statement> preludeStatements = astLookupValue.getAST() == null
         ? ImmutableList.<Statement>of() : astLookupValue.getAST().getStatements();
 
     // Load the BUILD file AST and handle Skylark dependencies. This way BUILD files are