Bring back missing modules warnings?

We have the ability to detect that modules are missing in the file-tree even though they are missing in the cabal file. Should we bring that back? Perhaps the errors from GHC are clear enough?

The reason given in the code, [here](https://github.com/commercialhaskell/stack/blob/190fa6e26c6ffb7d541f3f191a13453728c224af/src/Stack/Package.hs#L944), is that

> the issue with this is it's noisy for modules like Paths_*

Perhaps we should have a heuristic for which modules we shouldn't warn about? It won't work for custom autogen stuff, but that seems like a fine tradeoff to me.