nixos/systemd: move NSS module logic to systemd module

We keep the conditional on only adding if nscd is enabled for now.

nixos/modules/system/boot/systemd.nix

@@ -827,6 +827,27 @@ in
 
     system.build.units = cfg.units;
 
+    # Systemd provides various NSS modules to look up dynamic users, locally
+    # configured IP adresses and local container hostnames.
+    # On NixOS, these can only be passed to the NSS system via nscd (and its
+    # LD_LIBRARY_PATH), which is why it's usually a very good idea to have nscd
+    # enabled (also see the config.nscd.enable description).
+    # While there is already an assertion in place complaining loudly about
+    # having nssModules configured and nscd disabled, for some reason we still
+    # check for nscd being enabled before adding to nssModules.
+    system.nssModules = optional config.services.nscd.enable systemd.out;
+    system.nssDatabases = mkIf config.services.nscd.enable {
+      hosts = (mkMerge [
+        [ "mymachines" ]
+        (mkOrder 1600 [ "myhostname" ] # 1600 to ensure it's always the last
+      )
+      ]);
+      passwd = (mkMerge [
+        [ "mymachines" ]
+        (mkAfter [ "systemd" ])
+      ]);
+    };
+
     environment.systemPackages = [ systemd ];
 
     environment.etc = let