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