Depend on vlc-nox for mod_vlc

mod_vlc needs the plugins from vlc-nox to do anything useful.

Currently mod_vlc segfaults without the plugins.  Even after that is
fixed, however, without any plugins mod_vlc can't do its job, so we'll
set a hard dependency here.  I could probably be talked into making
this a Recommends rather than Depends.

FS-6124
Suggest vlc-dbg for mod_vlc

This helps to reason about any core dumps that come from libvlc /
mod_vlc.