All stages must be able to handle an input_dir argument, as we now
either pass it to all or none for agiven run. Simply set it to
'None' if it is not provided.
Signed-off-by: Tom Gundersen <teg@jklm.no>
Stages should be as stateless as possible. Don't provide an easy way out
of that.
Only the dnf stage used stage to save the dnf cache. That's only useful
during development and can be solved by pointing to a local repo mirror.
Rather than treating the dnf-cache specially, give each stage its
own state directory that they can reuse. This should obviously be
used with care by the stages in order to make the builds
reproducible.
