Indirect lookup of directive sections/clauses.

Per [bug 68278082](https://www.pivotaltracker.com/story/show/68278082).  Problem traced to the object list parameter (second in `handle_converter_csv_export`) being empty, which was due to `directive.sections` lookup returning empty list, possibly related to [PR 1135](https://github.com/reciprocity/ggrc-core/pull/1135).

It's still possible to find the related sections, by explicitly looking up the `directive_sections` connector, and that is what is used here.