dbusServices: Port to ESM

We want to replace gjs' custom (and now legacy) imports system
with standard EcmaScript modules: JS developers are already
familiar with them, they have better tooling support and using
standard features over non-standard ones is generally the right
thing to do.

Our D-Bus services are separate from the main process, and thus
can be ported separately (except for the few imports that are
shared with the main process' code base).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2786>

js/dbusServices/screensaver/main.js

@@ -1,11 +1,10 @@
-/* exported main */
+import {DBusService} from './dbusService.js';
+import {ScreenSaverService} from './screenSaverService.js';
 
-const { DBusService } = imports.dbusService;
-const { ScreenSaverService } = imports.screenSaverService;
-
-function main() {
+/** @returns {void} */
+export async function main() {
     const service = new DBusService(
         'org.gnome.ScreenSaver',
         new ScreenSaverService());
-    service.run();
+    await service.runAsync();
 }