The options at `systemd.network` (`links`, `netdevs` and `networks`) are
directly mapped to the three different unit types of `systemd-networkd(8)`.
However there's also the option `systemd.network.units` which is
basically used as a container for generated unit-configs that are linked
to `/etc/systemd/networkd`[1].
This should not be exposed to the user as it's unclear whether or not it
should be used directly which can be pretty confusing which is why I decided to
declare this option as internal (including all sub-options as `internal`
doesn't seem to be propagated to submodules).
[1] 9db75ed88f/nixos/modules/system/boot/networkd.nix (L933-L937)
60f2c59471