ostbuild: Lots of stuff

ostbuild: Lots of stuff

* Only create one build commit which contains multiple artifact trees,
  rather than one per artifact.  This is atomic.  We can use the new
  compose syntax like foo:/devel to slice out the /devel tree.
* Create the minimal buildroot for each component by composing the
  previous components in the build order, instead of continually
  updating one big tree.
* Ensure the artifact builder gets empty directories in /etc