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.

tests/modules/dbus/default.nix

@@ -0,0 +1,3 @@
+{
+  dbus-packages = ./packages.nix;
+}

tests/modules/dbus/packages.nix

@@ -0,0 +1,21 @@
+{ pkgs, config, ... }:
+let
+  inherit (config.lib.test) mkStubPackage;
+in
+{
+  dbus.packages = [
+    (mkStubPackage {
+      name = "test";
+      buildScript = ''
+        mkdir -p $out/share/dbus-1/services
+        printf '%s' test > $out/share/dbus-1/services/test.service
+      '';
+    })
+  ];
+
+  nmt.script = ''
+    serviceFile=home-files/.local/share/dbus-1/services/test.service
+    assertFileExists $serviceFile
+    assertFileContent $serviceFile "${pkgs.writeText "expected" "test"}"
+  '';
+}