In https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/merge_requests/153
the user target names for GSD components has been renamed for example
from `gsd-a11y-settings.target` to `org.gnome.SettingsDaemon.A11ySettings.target`,
and nowadays `gsd-*.target` are just symbolic links of `/dev/null` and will be
removed in the future.
At the same time, as mentioned in d27212d466,
we are adding `systemd.user.targets.<name>.wants` stuff here only because
systemd.packages doesn't pick the .wants directories. Nowadays those GSD components
are managed in `/etc/systemd/user/gnome-session@gnome.target.d/gnome.session.conf`
so it should be safe to remove them.
In this commit we also try to pick up those new .wants directories, see also
https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/blob/41.0/plugins/meson.build#L57
Result of `cd /nix/store/iqzy2a6wn9bq9hqx7pqx0a153s5xlnwp-gnome-settings-daemon-41.0; find | grep wants`:
```
./share/systemd/user/gnome-session-x11-services-ready.target.wants
./share/systemd/user/gnome-session-x11-services-ready.target.wants/org.gnome.SettingsDaemon.XSettings.service
./share/systemd/user/gnome-session-x11-services.target.wants
./share/systemd/user/gnome-session-x11-services.target.wants/org.gnome.SettingsDaemon.XSettings.service
```
Result of `cd /nix/store/armzljlnsvc1gn0nq0bncb9lf8fy32zy-gnome-settings-daemon-3.34.0; find | grep wants`:
```
./lib/systemd/user/gnome-session-initialized.target.wants
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-a11y-settings.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-color.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-datetime.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-power.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-housekeeping.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-keyboard.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-media-keys.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-screensaver-proxy.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-sharing.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-sound.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-smartcard.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-wacom.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-print-notifications.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-rfkill.target
./lib/systemd/user/gnome-session-initialized.target.wants/gsd-wwan.target
./lib/systemd/user/gnome-session-x11-services.target.wants
./lib/systemd/user/gnome-session-x11-services.target.wants/gsd-xsettings.target
```
When I designed `mkShell`, I didn't have a good idea of what the output
should look like and so decided to make the build fail. In practice,
this causes quite a bit of confusion and complications because now the
shell cannot be part of a normal package set without failing the CI as
well.
This commit changes that build phase to record all the build inputs in a
file. That way it becomes possible to build it, makes sure that all the
build inputs get built as well, and also can be used as a GC root.
(by applying the same trick as #95536).
The documentation has also been improved to better describe what mkShell
does and how to use it.
