To check whether a given module has been imported, we do the following:
From the list of all qualified names we extract the distinct module
names to a list of module names.
Then we check whether the given module name is in this list of module
names.

Test Plan: make test TEST=T14225

Reviewers: mpickering, hvr, monoidal, osa1, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #14225

Differential Revision: https://phabricator.haskell.org/D5331