Because:
* `go-bootstrap` is a native build input of go, so it needs to have
an offset of -1. Otherwise, e.g. when building a go cross-compiler,
it will try to make go-bootstrap a cross-compiler too.
* have to specify `buildPackages` for the `stdenv` override, otherwise
`buildPackages.stdenv` will be the same as `pkgs.gcc8Stdenv`.
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.
