libpulsecommon is a private library only for use within pulseaudio, so
lets move it into a private directory
[ed: from discussion on IRC, while it looks redundant to have modules in
lib/pulse-$VER and private libraries in $pkglibdir, this is actually
desirable. For example, a multi-architecture libpulse would have a
$pkgdir per architecture (/usr/lib/<arch>/pulseaudio) whereas you'd
still want the server-specific modules in /usr/lib/pulse...) -- Arun]