Add a warning for sections that are not on the top level.

Fixes #712.

Since sections can only be at the top level of the .cabal file, we should warn the user if we find something that looks like a section anywhere else. Caveat: This might not be the best place to handle this. We could also handle this in the mkField or ifelse functions in the ParseUtils module, but that would require a more substantial/invasive change.