Export findImportUsage and ImportDeclUsage
Reviewers: bgamari, alpmestan
Reviewed By: alpmestan
Subscribers: alpmestan, rwbarton, thomie, carter
GHC Trac Issues: Trac #15335
Differential Revision: https://phabricator.haskell.org/D4927
(cherry picked from commit 2b1adaa7817c453df868d928312a9a99a0481eb1)