For some reason having a file called `VERSION` in the package root directory
appears to be interfering with the build process:
```
Compiling Fl_cocoa.mm...
In file included from Fl_cocoa.mm:39:
In file included from ../FL/x.H:32:
In file included from ../FL/mac.H:50:
[...]
In file included from /nix/store/xhvrrnz8n6dxizgig46ijh59m7mfy261-libcxx-11.1.0-dev/include/c++/v1/cstddef:37:
../version:1:1: error: expected unqualified-id
1.4.0
^
```
By removing the file, the build succeeds.
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.
