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.
This is a mostly cosmetical commit, in the sense it doesn't change the contents
of any package, but reorganizes the overall Nixpkgs expressions.
Terminal emulators are an ubiquitous tool for any Unix user; even the beginners
are routinely familiarized to it. And, manifestly, there are many
implementations of terminal emulators out there, from those traditionally made
in C and C++ to those written in Haskell and Go.
Terminal emulators deserve more highlight. This commit does that by creating a
category for them.
