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.
Properly separate the build and install phases for better nix shell
hacking when using `buildPhase` -- and also fix a bug where
`$NIX_BUILD_CORES` wasn't properly passed to `bmake`.
Signed-off-by: Austin Seipp <aseipp@pobox.com>
