Split the session helper into a separate sub-package
    
    The org.freedesktop.Flatpak user D-Bus service isn't just used by
    flatpak(1) or applications running as Flatpaks. It's also used by
    toolbox(1) for similar reasons:
     * To keep various configuration files inside the container
       synchronized with the host
     * To let the container request certain commands to be run on the host
    
    Since the org.freedesktop.Flatpak D-Bus is a standalone utility that
    doesn't itself require the main flatpak package to work, this reduces
    the dependency footprint of toolbox for those who care about such
    things. eg., Fedora CoreOS.
    
    Add a versioned conflict on the old unified main flatpak package to
    resolve cases where the session-helper sub-package is requested to be
    installed when the old unified main package is already present. See:
    https://docs.fedoraproject.org/en-US/packaging-guidelines/Conflicts