Fix #11624, cannot declare hs-boot if already one in scope.

I'm not sure if this fix is the "right way" to do it, but
it solves the proximal problem, which is that lookupBindGroupOcc
was picking out the wrong renaming for hs-boot signatures,
which then lead to an interface file error.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1963

GHC Trac Issues: #11624