Move sky-box,-button,-checkbox,-radio out of their directories

examples/browser/browser.sky

 #!mojo mojo:sky_viewer
 <!--
 // Copyright 2014 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.
 -->
 <sky>
 
 <import src="/gen/examples/browser/browser_host.mojom.sky" as="example" />
 
 <import src="/sky/framework/embedder.sky" as="embedder" />
+<import src="/sky/framework/sky-box.sky"/>
 <import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
 <import src="/sky/framework/sky-input.sky" />
-<import src="/sky/framework/sky-box/sky-box.sky"/>
 
 <sky-element name="browser-panel">
 <template>
   <style>
   div {
     display: flex;
   }
   </style>
 
   <sky-input id="text" value="{{ url }}" on-keydown="handleKeyDown" />
 </template>
 <script>
 module.exports = class extends SkyElement {
   created() {
     this.embedder = embedder.connectToEmbedderService(example.BrowserHost)
     this.url = "http://";
   }
 
   handleKeyDown(event) {
     if (event.key == 0xD)
       this.embedder.navigateTo(this.url);
   }
 }.register();
 </script>
 </sky-element>
 
 <browser-panel />
 </sky>