build: Automatically expand pkgconfig dependencies

In more complex packages, the provided .pc file can tell us a lot
about the dependencies required to build using this package. For
instance, a gtk3-devel package provides several .pc files, which
have both public and private requirements. We expand the provided
information to our own package set to add automatic dependencies
to the -devel package, ensuring other packages can cleanly build
when using this -devel package.