Add support for not having koji volume mounted locally
    
    With this patch, Pungi can be configured with a local directory to be
    used as a cache for RPMs, and it will download packages from Koji over
    HTTP instead of reading them from filesystem directly.
    
    The files from the cache can then be hardlink as usual.
    
    There is locking in place to avoid different composes running at the
    same time to step on each other.
    
    This is now supported for RPMs only, be it real builds or scratch
    builds.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>