support selecting the pulse server implementation during installation

Currently, all packages that require a pulse server explicitly depend on the `pulseaudio` package, thus making pacman just use it as a default implementation without asking. However, this could break for e.g. people selecting `wireplumber` and wanting to use `pipewire-pulse` as their pulse server. I think making all pulse server implementations provide `PULSE-SERVER` or so and letting other packages depend on this virtual package could fix this. Moreover, it allows users to select the desired one first time during the installation, avoiding unnecessary switching afterwards.