taffybar: init with-packages wrapper

This provides a wrapper around taffybar, in a fashion similar to how
package xmonad.

pkgs/applications/window-managers/taffybar/default.nix

···
+{ stdenv, ghcWithPackages, xmessage, makeWrapper, packages ? (x: []) }:
+
+let
+taffybarEnv = ghcWithPackages (self: [ self.taffybar ] ++ packages self);
+in stdenv.mkDerivation {
+  name = "taffybar-with-packages";
+
+  nativeBuildInputs = [ makeWrapper ];
+
+  buildCommand = ''
+    mkdir -p $out/bin
+    makeWrapper ${taffybarEnv}/bin/taffybar $out/bin/taffybar \
+      --set NIX_GHC "${taffybarEnv}/bin/ghc"
+  '';
+}

pkgs/top-level/all-packages.nix

···
     enableXft = true;
   };
 
+  taffybar = callPackage ../applications/window-managers/taffybar {
+    inherit (haskellPackages) ghcWithPackages;
+  };
+
   tagainijisho = callPackage ../applications/office/tagainijisho {};
 
   tahoelafs = callPackage ../tools/networking/p2p/tahoe-lafs {