As per systemd.special(7)[0] graphical-session-pre.target is strictly
for units that set up things for a graphical session. Most notably,
these are usually started *before* the compositor/session is actually
ready.
While Home Manager's current implementation of graphical-session.target
allows these units to work regardless of what systemd.special(7)
specifies, other setups like ones with uwsm[1] do not allow these units
to start properly.
[0]: https://www.freedesktop.org/software/systemd/man/latest/systemd.special.html#graphical-session-pre.target
[1]: https://github.com/Vladimir-csp/uwsm
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
