Fix support for extending Dhall package set

* Fix `dhallPackages` to be defined using `callPackage` instead of
  `callPackages`

  This enables the `pkgs.dhallPackages.override` method

* Export `pkgs.dhallPackages.callPackage`

  This is also necessary in order to easily extend `pkgs.dhallPackages` with
  new packages

pkgs/top-level/all-packages.nix

@@ -10615,7 +10615,7 @@ in
 
   dhall-text = haskell.lib.justStaticExecutables haskellPackages.dhall-text;
 
-  dhallPackages = callPackages ./dhall-packages.nix { };
+  dhallPackages = callPackage ./dhall-packages.nix { };
 
   duktape = callPackage ../development/interpreters/duktape { };
 

pkgs/top-level/dhall-packages.nix

@@ -19,6 +19,7 @@ let
 
     in
       { inherit
+          callPackage
           buildDhallPackage
           buildDhallGitHubPackage
           buildDhallDirectoryPackage