Fix hpack support

Author: hamishmack

modules/cabal-project.nix

@@ -116,6 +116,10 @@ in {
       type = attrsOf (functionTo attrs);
       default = {};
     };
+    supportHpack = mkOption {
+      type = bool;
+      default = false;
+    };
 
     # Used by mkCabalProjectPkgSet
     pkg-def-extras = mkOption {

test/cabal-hpack/default.nix

@@ -15,6 +15,7 @@ let
   project = project' {
     inherit compiler-nix-name evalPackages;
     src = testSrc "cabal-hpack";
+    supportHpack = true;
     inherit modules;
   };