feat: pass cwd to formatters (refs eslint/rfcs#57) #13392
This is ready for review.
The rebase turned out to be a little more complicated than expected because this was previously using the in-memory filesystem that we've since stopped using. I created a fixture instead. There were a few tests that use the formatter fixtures directory as a target, and I had to update those with the new file.
mysticatea's commits are covered by the old CLA, so I've hidden the comment from the CLA bot.