meson: find dependencies only when necessary + some other dependency-related fixes
This allows finding dependencies conditionally, only when the enabled features actually need them.
Examples of scenarios that are being fixed:
- compile with -Dv4l2=false -> libv4l2 doesn't need to be installed in the system
- compile with -Dbluez5=true -> bluez and sbc MUST be installed in the system
- compile with -Dspa-plugins=false and -Dpipewire-alsa=true -> alsa must be installed and found
- compile with -Dpipewire-pulseaudio=false and -Dgstreamer=false -> glib doesn't need to be installed
This also removes some dependencies that were being found but not used, like X11, libva and avfilter