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.
The `dev` output is empty anyway.
The problem is that it interacts badly with other parts of python and
stdenv infrastructure. In particular, before this patch it installs
code into `out` output (with only generated `nix-support` in `dev`),
but `makePythonPath` then uses `propagatedBuildInputs` to generate
`PYTHONPATH` while stdenv selects `dev` outputs for
`propagatedBuiltInputs`. This results in `makePythonPath` linking to
the empty `dev` output in `PYTHONPATH`.
This reverts a piece of commit 28299f669a.
