Squashed 'hadrian/' changes from fa3771fe..4499b294

4499b294 Follow GHC changes
8fd68186 Add ways to build hadrian using nix
e5c7a29c Do not depend on the in-tree filepath library
9dd7ad2a Fix dependencies
49718439 Bring mtl dependency back
6c5f5c9b Minor clean up of Hadrian dependencies
9aff81d4 Fix Windows build
fa95caa8 Unbreak `cabal new-build`

git-subtree-dir: hadrian
git-subtree-split: 4499b294