Moves the files imported from `bootc-image-builder` to the appropriate
path under `pkg/` so they can be imported reverse. Also fix up the
import paths that are in these files to refer to their new locations.
Signed-off-by: Simon de Vlieger <supakeen@redhat.com>
The getContainerSize() was not using some of the modern go helpers.
So let's use `exec.Command().Output()` and introduce a new
`util.OutputErr()` helper that will be able to also show stderr to
the user if the Output() call returns an error.