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

mojo/dart/packages/mojom/bin/mojom.dart

 // 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 'dart:io';
 
 import 'package:mojom/src/command_runner.dart';
 import 'package:mojom/src/utils.dart'
     show CommandLineError, GenerationError, FetchError;
 
 main(List<String> arguments) async {
   var commandRunner = new MojomCommandRunner();
   try {
     return await commandRunner.run(arguments);
   } on CommandLineError catch (e) {
     stderr.writeln("$e\n${commandRunner.usage}");
   } on GenerationError catch (e) {
     stderr.writeln("$e\n${commandRunner.usage}");
   } on FetchError catch (e) {
     stderr.writeln("$e\n${commandRunner.usage}");
   }
 }