dbus: Create with pacakges options (#7064)

This adds a module for dbus with only one option, `packages`.
The `dbus.packages` options allows users to specify packages to have
their dbus service files (from `/share/dbus-1/services`) linked to the
users dbus services directory (`$XDG_DATA_HOME/dbus-1/services/`),
effectively enabling the services.

modules/dbus.nix

@@ -0,0 +1,36 @@
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}:
+let
+  cfg = config.dbus;
+in
+{
+  meta.maintainers = [ lib.hm.maintainers.rosuavio ];
+
+  options.dbus = {
+    packages = lib.mkOption {
+      type = with lib.types; types.listOf types.package;
+      default = [ ];
+      description = ''
+        Packages whose D-Bus configuration files should be included in
+        the configuration of the D-Bus session-wide message bus. Specifically,
+        files in «pkg»/share/dbus-1/services will be included in the user's
+        $XDG_DATA_HOME/dbus-1/services directory.
+      '';
+    };
+  };
+
+  config = {
+    xdg.dataFile."dbus-1/services" = {
+      recursive = true;
+      source = pkgs.symlinkJoin {
+        name = "user-dbus-services";
+        paths = cfg.packages;
+        stripPrefix = "/share/dbus-1/services";
+      };
+    };
+  };
+}