Port "auxiliary stuff" to ESM

The various D-Bus services, the Extensions app and the portal helper share very little code with the main code base. That makes them a good starting point for transitioning to standard modules, as the change is largely self-contained, and the few shared bits can keep using legacy imports until the main code base is ported.

This also allows us to do the interesting bits (mainly related to how stuff is bootstrapped) without drowning them in thousands of lines of boring boilerplate (replacing "imports.foo" with "import from './foo.js'").