This makes it easier to support a wider variety of .desktop session files. In
particular this makes it possible to use both the «legacy» sessions and upstream
session files.
We separate `xsession` into two parts, `xsessionWrapper` and `xsession`.
`xsessionWrapper` sets up the correct environment and then lauches the session's
Exec command (from the .desktop file), falling back to launching the default
window/desktopManager through the `xsession` script (required by at least some
nixos tests).
`xsession` then _only_ handles launching desktop-managers/window-managers defined
through `services.xserver.desktopManager.session`.
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