cmake: Depend entirely on pkg-config if found for VPL

Released versions of VPL have a pkg-config that orders dependencies
before the main VPL library. This causes our library searching with
pkg-config hints to instead search for the first dependency of VPL.

Instead just import the pkg-config definition directly and use it. This
is a bit more correct as all the dependencies will be correctly linked.

Also install the library since the debian package doesnt seem to depend
on the library itself...

fixes #9573