Add buildDhall*Package support for generating documentation

The `buildDhall{Directory,GitHub}Package` utilities now take an
optional `document` argument for generating documentation using
`dhall-docs`.  The documentation is stored underneath the `./docs`
subdirectory of the build product.

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

@@ -14,12 +14,17 @@ lib.makePackageOverridable
     , src
     , # The file to import, relative to the root directory
       file ? "package.dhall"
+      # Set to `true` to generate documentation for the package
+    , document ? false
     }:
 
-    buildDhallPackage {
-      inherit name dependencies source;
+    buildDhallPackage
+      ( { inherit name dependencies source;
 
-      code = "${src}/${file}";
-    }
+          code = "${src}/${file}";
+
+        }
+      // lib.optionalAttrs document { documentationRoot = src; }
+      )
   )