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

mojo/dart/packages/mojo_apptest/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("mojo_apptest") {
   libs = [ "lib/apptest.dart" ]
   sources = [
     "pubspec.yaml",
   ]
   deps = [
-    "//mojo/public/dart/mojo",
+    "//mojo/dart/packages/mojo",
     "//mojo/public/dart/third_party",
   ]
 }