Issue with switching from generated files back to normal files.

If you rename a module from `Foo.hsc` to `Foo.hs`, cabal will keep recompiling the previously generated `Foo.hs` rather than take the one in your source directory. It seems the only way I have to get out of this bad state is to nuke `dist-newstyle` and try again.

It seems that our goal of a 'stateless' cache isn't quite being achieved here. I presume this happens with other preprocessors.