Without the change the build fails on `master` as
https://hydra.nixos.org/build/249086772:
In file included from /build/source/submodules/rocksdb/table/block_based/data_block_hash_index.cc:9:
/build/source/submodules/rocksdb/table/block_based/data_block_hash_index.h:65:7: error: 'uint8_t' does not name a type
65 | const uint8_t kNoEntry = 255;
| ^~~~~~~
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.
