Summary:
This is a fixup of https://phabricator.haskell.org/D233

The only difference is in findTFiles (first commit), which
previously broke Windows runner; now I translated literally
instead attempting to improve it, and checked it works.

Test Plan:
I used validate under 2,3 on Linux and under 2 on msys2.
On Windows I've seen a large number of failures, but they don't
seem to be connected with the patch.

Reviewers: hvr, simonmar, thomie, austin

Reviewed By: austin

Subscribers: thomie, carter, ezyang, simonmar

Differential Revision: https://phabricator.haskell.org/D310

GHC Trac Issues: #9184