This reverts commit 7fea7121.
It turns out that on a newly added MultiLayerModules test
case it gets very expensive to union the transitive module
sets while preserving determinism.

Fortunately, we can just sort to restore determinism
when converting imp_dep_mods to a list.

Test Plan: ./validate

Reviewers: simonmar, austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3577