Change idiom for overriding Dhall package version

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.

pkgs/development/interpreters/dhall/build-dhall-github-package.nix

@@ -1,7 +1,7 @@
 { buildDhallPackage, fetchFromGitHub, lib }:
 
 # This function is used by `dhall-to-nixpkgs` when given a GitHub repository
-lib.makeOverridable
+lib.makePackageOverridable
   ( { # Arguments passed through to `buildDhallPackage`
       name
     , dependencies ? []