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.
"platforms.gnu" has been linux-only since at least 17.03:
$ nix eval -f channel:nixos-17.03 lib.platforms.gnu
[ "i686-linux" "x86_64-linux" "armv5tel-linux" "armv6l-linux" "armv7l-linux" "aarch64-linux" "mips64el-linux" ]
Unlike platforms.linux, platforms.gnu indicates "must use glibc"
which for the most part is not intended.
Replacing platforms.gnu with platforms.linux would be the same "today"
but let's err on preserving existing behavior and be optimistic
about platforms these packages work on.
* Remove package name
* Start with upper case letter
* Remove trailing period
Also reword some descriptions and move some long descriptions to
longDescription.
I'm not touching generated packages.
