libiconv is already defined per-platform. The actual libiconv library
won't be built on platforms like Linux where it doesn't need to be, so
there's no need to maintain a separate platform list here.
Fixes pkgsCross.x86_64-freebsd.pkg-config.
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.
Package attrs should map to the pname. It's confusing otherwise.
pkg-config is also the name used by most distributions:
Debian, Homebrew, Guix, OpenSuse
other distributions like FreeBSD ports, Fedora and Arch Linux call it
"pkgconf"
Waiting a few releases to make a tree-wide change to make it easier to
back-port things.
