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

examples/wm_flow/wm/window_frame.sky

 #!mojo mojo:sky_viewer
 <!--
 // 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.
 -->
 <sky>
 <import src="/gen/examples/wm_flow/wm/window_frame_host.mojom.sky" as="example" />
 <import src="/sky/framework/embedder.sky" as="embedder" />
-<import src="/sky/framework/sky-button/sky-button.sky"/>
-<import src="/sky/framework/sky-checkbox/sky-checkbox.sky"/>
+<import src="/sky/framework/sky-button.sky"/>
+<import src="/sky/framework/sky-checkbox.sky"/>
 <import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement"/>
 
 <style>
 /* FIXME: Hack until the requirement of a single root element is lifted. */
 sky { height: 100%; }
 </style>
 
 <sky-element name="window-frame">
 <template>
   <style>
@@ skipping 53 matching lines @@
   handleBegoneClick(event) {
     this.embedder.closeWindow();
   }
 }.register();
 </script>
 </sky-element>
 
 <window-frame />
 
 </sky>