The `Ord Module` instance currently uses `Unique`s for comparison.
We don't want to use the `Unique` order because it can introduce
nondeterminism.
This switches `Ord ModuleName` and `Ord UnitId` to use lexicographic
ordering making `Ord Module` deterministic transitively.

I've run `nofib` and it doesn't make a measurable difference.

See also Note [ModuleEnv determinism and performance].

This fixes #12191 - the regression, that the previous version of this
patch had.

Test Plan:
./validate
run nofib: P112

Reviewers: simonmar, bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2354

GHC Trac Issues: #4012, #12191