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
