pythonPackages.pkgconfig: use same setupHook as pkgconfig

This setupHook is necessary for actually using pkgconfig. Without this
setupHook, you need to put both pythonPackages.pkgconfig and pkgconfig
into your buildInputs, just to get the setuphook of the latter.

Previously, pythonPackages.pkgconfig depended on pkgconfig as a
propagatedBuildInput, so this setupHook was propagated down to
dependents. That was changed to a regular nativeBuildInput, so now
this setupHook change is necessary for packages to only depend on
pythonPackages.pkgconfig.

A possible alternative is extracting out a common pkgconfigHook
package and having both pythonPackages.pkgconfig and pkgconfig depend
on that with propagatedBuildInputs. That might be necessary to avoid
the buildHook running twice?