stdenv-setup: Make the package accumulators associative arrays instead of strings

This is generally cleaner: less eval, less worrying about separators,
and probably also faster. I got the idea from that python wrapper
script.

pkgs/development/haskell-modules/generic-builder.nix

@@ -211,7 +211,7 @@ stdenv.mkDerivation ({
     configureFlags="${concatStringsSep " " defaultConfigureFlags} $configureFlags"
 
     # nativePkgs defined in stdenv/setup.hs
-    for p in $nativePkgs; do
+    for p in "''${!nativePkgs[@]}"; do
       if [ -d "$p/lib/${ghc.name}/package.conf.d" ]; then
         cp -f "$p/lib/${ghc.name}/package.conf.d/"*.conf $packageConfDir/
         continue