modules/pkgconfig: get rid of modifications to the BuildTarget objects
modules/pkgconfig: get rid of modifications to the BuildTarget objects

This would tack on extra attributes for meta data tracking. Do that with
our own datastructures instead

pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson