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.
* 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.
Robert Scott