Overrides often inherit pname from the overriden package, and if they
get the already-prefixed version, defaulting the systems list to [pname]
produces wrong (and unintuitive) results. So we keep the prefix in the
store paths for clarity, and nix-env sees disambiguated names with -i/-u
(which should not be used anyway); but overrides won't be broken.
The previous approach of trying to make both the `override` mechanism from
`mkDerivation` and the `overrideScope'` mechanism from `newScope` work together
resulted in hard to understand code, and there was a bug where once overridden
packages would lose the changes on next override with `packageOverrides`.
It's not ideal still, because Lisps created by `mkDerivation` will lose their
`pkgs` after using `override`.
The previous approach of manually repeating a complex pattern inside Common Lisp
implementation package declarations was fragile and hard to change. After
reading python and lua modules code in Nixpkgs, I was able to come up with
something better.
The function `wrapLisp` doesn't need to be inside package declarations so all
the code for wrapping Lisps can be in `all-packages.nix`.
This works by wrapping the `override` function created from `mkDerivation` to
accept a new argument `packageOverrides`.
One problem with this is that `override.__functionArgs` disappears. But one can
look at the source code of a package to discover what can be overridden.
