lib/options: refactor mkPackageOption

      }:

      let

        name' = if isList name then last name else name;

      in mkOption ({

        type = with lib.types; (if nullable then nullOr else lib.id) package;

        description = "The ${name'} package to use."

          + (if extraDescription == "" then "" else " ") + extraDescription;

      } // (if default != null then let

        default' = if isList default then default else [ default ];

        defaultPath = concatStringsSep "." default';

        defaultText = concatStringsSep "." default';

        defaultValue = attrByPath default'

          (throw "${defaultPath} cannot be found in ${pkgsText}") pkgs;

      in {

          (throw "${defaultText} cannot be found in ${pkgsText}") pkgs;

        defaults = if default != null then {

          default = defaultValue;

        defaultText = literalExpression ("${pkgsText}." + defaultPath);

      } else if nullable then {

          defaultText = literalExpression ("${pkgsText}." + defaultText);

        } else optionalAttrs nullable {

          default = null;

      } else { }) // lib.optionalAttrs (example != null) {

        };

      in mkOption (defaults // {

        description = "The ${name'} package to use."

          + (if extraDescription == "" then "" else " ") + extraDescription;

        type = with lib.types; (if nullable then nullOr else lib.id) package;

      } // optionalAttrs (example != null) {

        example = literalExpression

          (if isList example then "${pkgsText}." + concatStringsSep "." example else example);

      });