fix lookup of document root when cabal install'ed (bug introduced in aa6...

...54698a81adb77b71acb9323590e64a6b8b318)
I think `cabalDataDir` is a misnomer here.  Also, in that specific case I think it would be nice to have something like:

```haskell
let dirForStaticFiles = cabalDataDir </> "www"
```