Before this change, a Dhall package like the Prelude would be
encoded as a record with one field per supported version. Then
downstream packages would specify which package to override
by selecting a different record field.

The problem with that approach is that it did not provide an
easy way to override a package to a version other than the default
ones supplied by Nixpkgs. Normally you would use the .override
method for this purpose, but the override method added by
buildDhall{Directory,GitHub}Package is clobbered by the
override method added by callPackage in
./pkgs/top-level/dhall-packages.nix.

The solution is to add a separate .overridePackage method which is
essentially the exact same as .override, except that it is no
longer clobbered by callPackage. This .overridePackage method
allows one to override the arguments supplied to
buildDhall{Directory,GitHub}Package, making it easier to specify
package versions outside of the ones supported by Nixpkgs..

This also includes a change to only build one (preferred) version of each
package (instead of multiple supported versions per package), in order to
minimize the maintenance burden for the Dhall package set.

I verified that all of the supported Dhall packages still build after this change.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.