nixos/systemd-lib: fix conflict with dbus.service.d directory

When a package contains a directory in one of the systemd directories
(like flatpak does), it is symlinked into the *-units derivation.
Then later, the derivation will try to create the directory, which
will fail:

mkdir: cannot create directory '/nix/store/…-user-units/dbus.service.d': File exists
builder for '/nix/store/…-user-units.drv' failed with exit code 1

Closes: #33233

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

@@ -2,9 +2,10 @@
 
 with lib;
 
-let cfg = config.systemd; in
-
-rec {
+let
+  cfg = config.systemd;
+  lndir = "${pkgs.xorg.lndir}/bin/lndir";
+in rec {
 
   shellEscape = s: (replaceChars [ "\\" ] [ "\\\\" ] s);
 
@@ -136,7 +137,13 @@ rec {
       for i in ${toString cfg.packages}; do
         for fn in $i/etc/systemd/${type}/* $i/lib/systemd/${type}/*; do
           if ! [[ "$fn" =~ .wants$ ]]; then
-            ln -s $fn $out/
+            if [[ -d "$fn" ]]; then
+              targetDir="$out/$(basename "$fn")"
+              mkdir -p "$targetDir"
+              ${lndir} "$fn" "$targetDir"
+            else
+              ln -s $fn $out/
+            fi
           fi
         done
       done
@@ -151,7 +158,7 @@ rec {
           if [ "$(readlink -f $i/$fn)" = /dev/null ]; then
             ln -sfn /dev/null $out/$fn
           else
-            mkdir $out/$fn.d
+            mkdir -p $out/$fn.d
             ln -s $i/$fn $out/$fn.d/overrides.conf
           fi
        else