Be able to set the servicedirs precedence
Is your feature request related to a problem? Please describe.
When developing and using a development version of a desktop environment such as Plasma and GNOME it is useful to have the distribution packages co-installed so if something goes terribly bad, you can always fallback to the distribution provided version. Right now, this can't be done since the system servicedirs are always loaded first.
Another use case is just to have GNOME-stable and GNOME-unstable co-installed and just choose one of the versions from the DM.
Describe the solution you'd like
Would be lovely to have a way of defining the precedence of servicedirs at least at launch time so display managers and desktop environment can set the precedence order when launching the session bus.
Describe alternatives you've considered
A static configuration would be good enough for developers but it wouldn't cut it if we want users to be able to easily test development versions of the DE.
Additional context
I opened an issue to dbus-broker where it was mentioned that his has been discussed before. Couldn't find anything in this gitlab instance though. David Rheinsberg also provided an alternative approach that will replace D-Bus service files with systemd units.