Nixpkgs support for dhall-to-nixpkgs

The motivation for this change is to enable a new Dhall command-line
utility called `dhall-to-nixpkgs` which converts Dhall packages to
buildable Nix packages.  You can think of `dhall-to-nixpkgs` as the
Dhall analog of `cabal2nix`.

You can find the matching pull request for `dhall-to-nixpkgs` here:

https://github.com/dhall-lang/dhall-haskell/pull/1826

The two main changes required to support `dhall-to-nixpkgs` are:

* Two new `buildDhall{Directory,GitHub}Package` utilities are added

  `dhall-to-nixpkgs` uses these in the generated output

* `pkgs.dhallPackages` now selects a default version for each package
  using the `prefer` utility

  All other versions are still buildable via a `passthru` attribute

pkgs/development/dhall-modules/Prelude.nix

@@ -1,26 +1,17 @@
-{ buildDhallPackage, fetchFromGitHub, lib }:
+{ buildDhallGitHubPackage, lib }:
 
 let
   makePrelude =
-    version:
-    lib.makeOverridable
-      ( { rev, sha256, file ? "package.dhall" }:
-          buildDhallPackage {
-            name = "Prelude-${version}";
+    version: { rev, sha256 }:
+      buildDhallGitHubPackage {
+        name      = "Prelude-${version}";
+        owner     = "dhall-lang";
+        repo      = "dhall-lang";
+        directory = "Prelude";
+        file      = "package.dhall";
 
-            code =
-              let
-                src = fetchFromGitHub {
-                  owner = "dhall-lang";
-                  repo  = "dhall-lang";
-
-                  inherit rev sha256;
-                };
-
-              in
-                "${src}/Prelude/${file}";
-          }
-      );
+        inherit rev sha256;
+      };
 
 in
   lib.mapAttrs makePrelude {