Make systemd-network-generator co-owned by -udev and -networkd
    
    ... (rhbz#2328723)
    
    The files systemd-networkd-generator generates are read by udev (.link files)
    and by networkd (.netdev, .netdev files). We can't move it to systemd-networkd
    subpackage only, because that would potentially break the corner case of people
    having systemd-udev installed and using the generator, but not systemd-networkd.
    And there is no dependency from systemd-networkd to systemd-udev. I think this
    is correct, because networkd can be used in containers without udev. But the
    generator is not useful without either of those two daemons, so let's move
    it to make the core package a bit lighter.