haskell generic-builder: Always use separate pkg db for custom setup

This decreases complexity and ensures setup dependencies are properly
specified with `setup-depends` as they should be. Testing will say if
this is a reasonable change.

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

@@ -166,14 +166,8 @@ let
     "--configure-option=--host=${hostPlatform.config}"
   ] ++ crossCabalFlags);
 
-  useSeparateSetupDb = setupHaskellDepends != [] || isCross || isGhcjs;
-
   setupCompileFlags = [
-    (optionalString (!coreSetup) "-${nativePackageDbFlag}=${
-      if useSeparateSetupDb
-      then "$setupPackageConfDir"
-      else "$packageConfDir"
-    }")
+    (optionalString (!coreSetup) "-${nativePackageDbFlag}=$setupPackageConfDir")
     (optionalString (isGhcjs || isHaLVM || versionOlder "7.8" ghc.version) "-j$NIX_BUILD_CORES")
     # https://github.com/haskell/cabal/issues/2398
     (optionalString (versionOlder "7.10" ghc.version && !isHaLVM) "-threaded")
@@ -270,10 +264,8 @@ stdenv.mkDerivation ({
     echo "Build with ${ghc}."
     ${optionalString (hasActiveLibrary && hyperlinkSource) "export PATH=${hscolour}/bin:$PATH"}
 
-  '' + (optionalString useSeparateSetupDb ''
     setupPackageConfDir="$TMPDIR/setup-package.conf.d"
     mkdir -p $setupPackageConfDir
-  '') + ''
     packageConfDir="$TMPDIR/package.conf.d"
     mkdir -p $packageConfDir
 
@@ -284,14 +276,13 @@ stdenv.mkDerivation ({
   # dependencies for the build machine.
   #
   # pkgs* arrays defined in stdenv/setup.hs
-  + (optionalString useSeparateSetupDb ''
+  + ''
     for p in "''${pkgsBuildBuild[@]}" "''${pkgsBuildHost[@]}" "''${pkgsBuildTarget[@]}"; do
       ${buildPkgDb nativeGhc.name "$setupPackageConfDir"}
     done
     ${nativeGhcCommand}-pkg --${nativePackageDbFlag}="$setupPackageConfDir" recache
-  '')
-
-    # For normal components
+  ''
+  # For normal components
   + ''
     for p in "''${pkgsHostHost[@]}" "''${pkgsHostTarget[@]}"; do
       ${buildPkgDb ghc.name "$packageConfDir"}