This impacts at least the order in which version macros are
generated. It's pretty hard to track what kind of nondeterminism
is benign and this should have no performance impact as the number
of packages should be relatively small.

Test Plan: ./validate

Reviewers: simonmar, austin, bgamari, ezyang

Reviewed By: ezyang

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2308

GHC Trac Issues: #4012