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.
Configure log still prints `checking for GST... yes`
and the new GST libs appear in RPATH.
wxGTK28 does't seem able to configure with non-legacy GST,
but it's almost unused... so I didn't touch it.
bc109b19c9a503948e0f88272224422c456ed5b9 on master created a separate
expression based on wxGTK30, but that expression was modified on staging
in f0733a07d5a1347eeee1fda81eee336fef50be66. Let's sync that.