Move implementation packages into
org.apache.sling.scripting.sightly.impl to leverage default non-export
behaviour of the bundle plugin and thus not exporting anything from the
bundle.
Reorganize API packages:

  * Move Use related stuff to .use
  * Move Render related stuff to .render
  * Move Extension related stuff to .extension
  * Move rest of .api to org.apache.sling.scripting.sightly