This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
Fluidsynth's systemd unit currently has a hard dependency on the
pulseaudio systemd service. Since fluidsynth can use other sound
services (e.g., pipewire-pulse), this should be configurable. This
commit adds the relevant option.
PR #2238
