Allow running sub-pipelines in Weaver.
    
    Some composes recently start using scm_dict in extra_files which makes
    this phase longer (around 3 minutes), but in fact it is possible to
    execute extra files phase together with gather phase.
    
    This commit allows that by introducing "sub-pipelines" in the Weaver
    phase.
    
    The saved time is interesting only for partial composes which do not
    run buildinstall phase, because the buildinstall phase takes much
    more time. But it might be useful even with buildinstall in case
    we will start caching boot.iso.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>