continuation of #109595
pkgconfig was aliased in 2018, however, it remained in
all-packages.nix due to its wide usage. This cleans
up the remaining references to pkgs.pkgsconfig and
moves the entry to aliases.nix.
python3Packages.pkgconfig remained unchanged because
it's the canonical name of the upstream package
on pypi.
Headers were disabled, probably because they caused a build error: the header files were being
copied to the store path of wxwidgets, but it failed with permission denied. With this commit,
the header files stay in the wxPython store path.
To point the builder of another derivation to the wxPython header files, some tweaking is
needed. E.g. for kicad: cmakeFlags = [ "-DCMAKE_CXX_FLAGS=-I${wxPython}/include/wx-3.0" ]
