After an extended debugging session with Simon Michael it turns out that Stack relies on direct parsing of interface files (via its own parsing implementation, https://github.com/commercialhaskell/hi-file-parser/blob/master/src/HiFileParser.hs). This is awful; interface files are not supposed to be a public interface. We should to figure out what information Stack needs from GHC to eliminate the use of interface files in this way and find another way to expose that information (perhaps the [extended dependency generation proposal](https://github.com/ghc-proposals/ghc-proposals/pull/245) is a step in the right direction?).