Do not wrap built-in function in JetLightClass (EA-38770)

Built-in top-level functions are located in jet.namespace class.
JetStandardClasses.isStandardClass() now checks that a given fqName starts with
"jet.", instead of searching it in the standard scope, to prevent this kind of
errors in the future

compiler/frontend/src/org/jetbrains/jet/lang/types/lang/JetStandardClasses.java

@@ -613,11 +613,6 @@ public class JetStandardClasses {
     }
 
     public static boolean isStandardClass(@NotNull FqName fqName) {
-        for (DeclarationDescriptor descriptor : getAllStandardClasses()) {
-            if (DescriptorUtils.getFQName(descriptor).equals(fqName.toUnsafe())) {
-                return true;
-            }
-        }
-        return false;
+        return JetStandardClasses.STANDARD_CLASSES_FQNAME.equals(fqName.parent());
     }
 }