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.
gts generate binaries during the build process that are used during the
build process. When building cross-platform, these binaries are not
executable as they are built with for the target platform instead of the
host platform. The boostrap build introduced here will first build gts
natively and then cross-platform, using the native binaries in the
cross-build.
