Cross-references in import/export lists when a whole module is imported/exported

[jinwoo]

#6 and #66 added support for xrefs in import/export lists. But when a whole module is imported or exported, rather than individual entities, it doesn't get xrefs. Fix it.

[jinwoo]

E.g. in

module Foo (module Foo.Internal) where

or

import Foo (module Bar)

, Foo.Internal or Bar must have xref links.