add containers-common-extra subpackage
    
    The extra subpackage will contain those dependencies for Podman and Buildah
    which are not required by Skopeo.
    
    There are also additional conditionals to handle dependencies on Fedora
    Server.
    
    Podman and Buildah will `Requires: containers-common-extra` going
    forward while Skopeo will ocntinue to `Requries: containers-common`.
    
    Also added comment about origin of SourceN files.
    
    Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>