Part 1 of #301908.
This renames the two versions of the Apple Public Source License seen in
nixpkgs; `apsl20` was often confused as being for the widely-used Apache
License 2.0.
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.
Undefined symbols for architecture x86_64:
"_CFNotificationCenterAddObserver", referenced from:
_main in main.o
"_CFNotificationCenterGetDistributedCenter", referenced from:
_main in main.o
"_OBJC_CLASS_$_NSTimer", referenced from:
objc-class-ref in main.o
objc-class-ref in x-screen.o
"_OBJC_EHTYPE_$_NSException", referenced from:
GCC_except_table25 in main.o
ld: symbol(s) not found for architecture x86_64
