Idea: a `pkg-config` module

[roberth]

A package that expose pkg-config "modules" (.pc files) should do a couple of things. In the current nixpkgs that would be

• add validatePkgConfig hook

In a future nixpkgs that may include

• declare the exposed pkg-config modules in a meta variable
• add a test to make sure that the declared pkg-config modules are present and valid in the output