Lint fails with unhelpful error message when not run in project root

[DanielG]

withMappedFile probably doesn't expect to be run outside project root (cc @lierdakil)

(Reported on 2017-04-30 via IRC by __monty__)

[lierdakil]

I'm pretty sure we always chdir to project root, don't we?

[DanielG]

I think we only do that in GmlT, that lint is just in GhcModT but I don't recall without looking at the code.