Reproducing case for: commercialhaskell/stack#2781

To repro: run ./repro.sh. (Imaginative, I know.)

./cabal-repro.sh will reproduce this using only Cabal-the-library (not Stack or cabal-install). It only uses Stack to find the location of GHC; you can comment out that bit if you have GHC 8.2.1 on your PATH already.