Apper can show this information if the backend provides it. (E.g., the old yum backend provided it.)

The recursive case is fairly complicated (does anybody actually use that?), but the non-recursive versions should be enough for Apper. There, I would just expect a full unfiltered list of what amounts to repoquery --requires foo resp. repoquery --whatrequires --alldeps foo. The first is just a hy_package_get_requires. For the second, we need a hy_query_filter_reldep_in(…, HY_PKG_REQUIRES, hy_package_get_provides(…)).