Rationalize Dart mojo and sky package structure

mojo/dart/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_package("apptest") {
   uses_pub = true
   sources = [
-    "apptest/apptest.dart",
+    "lib/apptest.dart",
+    "pubspec.yaml",
   ]
   deps = [
     "//mojo/public/dart",
   ]
 }
+
+dart_pkg("apptest_pkg") {
+  sources = [
+    "lib/apptest.dart",
+    "pubspec.yaml",
+  ]
+}