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.

nixos/modules/system/boot/systemd-lib.nix

@@ -147,7 +147,13 @@ in rec {
       done
 
       # Symlink all units provided listed in systemd.packages.
-      for i in ${toString cfg.packages}; do
+      packages="${toString cfg.packages}"
+
+      # Filter duplicate directories
+      declare -A unique_packages
+      for k in $packages ; do unique_packages[$k]=1 ; done
+
+      for i in ''${!unique_packages[@]}; do
         for fn in $i/etc/systemd/${type}/* $i/lib/systemd/${type}/*; do
           if ! [[ "$fn" =~ .wants$ ]]; then
             if [[ -d "$fn" ]]; then