rwbarton authored Feb 14, 2017 and Ben Gamari committed Feb 14, 2017

Preparation for #13102, which needs to add more logic to
runTcInteractive, which would need to be duplicated in deSugarExpr.

In order to break an import cycle, I had to move
"Dependency/fingerprinting code" to a new module
DsUsage; which seems sensible anyways.

Test Plan: validate

Reviewers: simonpj, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie, snowleopard

Differential Revision: https://phabricator.haskell.org/D3125