nixos/systemd-lib: don't fail on systemd.packages duplicates

In some cases like we've noticed in https://github.com/NixOS/nixpkgs/issues/76169,
having duplicate packages in systemd.packages like
```
systemd.packages = [ gnome-shell gnome-shell gnome-session ];
```
breaks.

Here we use an associative array to ensure no
duplicate paths when we symlink all the units listed
in systemd.packages.