Since glibc-2.38[1], `strcasestr(3)` is available by default, not just
when using _GNU_SOURCE. This means that the check whether this function
is available in `brickd`'s build is bogus now since it bases this
decision on __APPLE__ and _GNU_SOURCE rather than __GLIBC__.
Failing Hydra build: https://hydra.nixos.org/build/230504252
[1] upstream commit 30e3ca78f92be44f46851557f54b47e789bd3416
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.
