Move mojo and mojom from mojo/public/dart to mojo/dart/packages

examples/dart/hello_world/hello/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//mojo/public/dart/rules.gni")
 
 dart_pkg("hello") {
   apps = [ [
         "dart_hello",
         "lib/main.dart",
       ] ]
   sources = [
     "pubspec.yaml",
   ]
   deps = [
+    "//mojo/dart/packages",
     "//mojo/public/dart",
   ]
 }