Improve the warning message of qualified unused imports.

Authored by sighingnow on Mar 19 2018, 10:58 AM.

Description

Improve the warning message of qualified unused imports.

Pretty-print unused imported names unqualified unconditionally to
make the warning message consistent for ambiguous/unambiguous
identifiers.

Signed-off-by: HE, Tao <sighingnow@gmail.com>

Test Plan: make test TEST="T14881"

Reviewers: bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie, carter

GHC Trac Issues: Trac #14881

Differential Revision: https://phabricator.haskell.org/D4461