Use the same directory layout for user-installed Haskell libraries that GHC

uses for its core libraries, so that these files integrate seamlessly into one
profile, living right next to each other. This change is eventually going to
simply our with-packages wrapper quite a bit.

pkgs/build-support/cabal/default.nix

@@ -121,7 +121,7 @@
               done
 
               echo "configure flags: $extraConfigureFlags $configureFlags"
-              ./Setup configure --verbose --prefix="$out" $extraConfigureFlags $configureFlags
+              ./Setup configure --verbose --prefix="$out" --libdir='$prefix/lib/$compiler' --libsubdir='$pkgid' $extraConfigureFlags $configureFlags
 
               eval "$postConfigure"
             '';
@@ -156,7 +156,7 @@
 
               ensureDir $out/bin # necessary to get it added to PATH
 
-              local confDir=$out/lib/ghc-pkgs/ghc-${ghc.ghc.version}
+              local confDir=$out/lib/ghc-${ghc.ghc.version}/package.conf.d
               local installedPkgConf=$confDir/${self.fname}.installedconf
               local pkgConf=$confDir/${self.fname}.conf
               ensureDir $confDir

pkgs/development/compilers/ghc/ghc-get-packages.sh

@@ -12,7 +12,7 @@ fi
 PATH="$2:$PATH"
 IFS=":"
 for p in $PATH; do
-  PkgDir="$p/../lib/ghc-pkgs/ghc-$version"
+  PkgDir="$p/../lib/ghc-$version/package.conf.d"
   for i in $PkgDir/*.installedconf; do
     # output takes place here
     test -f $i && echo -n " $prefix$i"

pkgs/development/compilers/ghc/with-packages.nix

@@ -51,7 +51,7 @@ stdenv.mkDerivation rec {
     echo "Linking selected packages and dependencies:"
 
     for currentPath in ${stdenv.lib.concatStringsSep " " allPackages}; do
-      currentPkgDir="$currentPath/lib/ghc-pkgs/ghc-${ghc.version}"
+      currentPkgDir="$currentPath/lib/ghc-${ghc.version}/package.conf.d/"
       # Check if current path is a Cabal package for the current GHC
       if test -d $currentPkgDir; then
         echo -n "Linking $currentPath "