Issue description

Thought I'd make an issue about this too, as the PR queue gets filled up pretty quickly.

Quoting from #53843 (note that sessionPackages was previously called extraSessionFilePackages):

Motivation for this change

Integrate services.xserver.displayManager.sessionPackages better with the existing session system.

services.xserver.displayManager.defaultSession

There's two ways of providing graphical sessions now:

1. displayManager.session via. desktopManager.session and
   windowManager.session
2. displayManager.sessionPackages

sessionPackages doesn't make a distinction between desktop and window
managers. This makes selecting a session provided by a package using
desktopManager.default nonsensical.

We therefor introduce displayManager.defaultSession which can select a session
from either displayManager.session or displayManager.sessionPackages.

It will default to desktopManager.default + windowManager.default as before.
If the dm default is "none" it will select the first provided session from
sessionPackages.