Move sky-*.sky into framework/elements

sky/examples/custom-element/example-element.sky

 <!--
 // 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 src="/sky/framework/sky-element.sky" />
+<import src="/sky/framework/elements/sky-element.sky" />
 <sky-element>
 <template>
   <style>
     div {
       height: 50px;
       width: 50px;
       background-color: green;
     }
   </style>
   <div>Hi</div>
 </template>
 <script>
 @Tagname('example')
 class ExampleElement extends SkyElement {
 }
 
 _init(script) => register(script, ExampleElement);
 </script>
 </sky-element>