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

Instead of having a separate directory for every Sky element, we should just
put them in the general framework directory. Once the framework is more
complicated, we'll probably want to organize it a bit better, but for now there
aren't enough files to justify having so many directories.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/874303003

examples/widgets/index.sky

@@ -4,12 +4,12 @@
 // found in the LICENSE file.
 -->
 <sky>
-<import src="/sky/framework/sky-box/sky-box.sky"/>
-<import src="/sky/framework/sky-button/sky-button.sky"/>
-<import src="/sky/framework/sky-checkbox/sky-checkbox.sky"/>
+<import src="/sky/framework/sky-box.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"/>
 <import src="/sky/framework/sky-input.sky"/>
-<import src="/sky/framework/sky-radio/sky-radio.sky"/>
+<import src="/sky/framework/sky-radio.sky"/>
 <sky-element name="widget-root">
 <template>
   <style>