refac: move package builder to a separate file

default.nix

@@ -0,0 +1,3 @@
+(import (fetchTarball https://github.com/edolstra/flake-compat/archive/master.tar.gz) {
+  src = builtins.fetchGit ./.;
+}).defaultNix

flake.nix

@@ -5,16 +5,7 @@
   };
 
   outputs = { nixpkgs, flake-utils, ... }:
-    let
-      inherit (builtins) readFile;
-
-      mkWd2 = { writeShellApplication, symlinkJoin, d2, ... }:
-        writeShellApplication {
-          name = "wd2";
-          runtimeInputs = [ d2 ];
-          text = (readFile ./wd2);
-        };
-    in
+    let mkWd2 = import ./wd2.nix; in
     {
       overlays = {
         default = final: prev: {

wd2.nix

@@ -0,0 +1,7 @@
+{ writeShellApplication, d2, ... }:
+
+writeShellApplication {
+  name = "wd2";
+  runtimeInputs = [ d2 ];
+  text = builtins.readFile ./wd2;
+}