`cabal new-build` should warn if it's building more than one package

Sometimes, the behaviour of `cabal new-build` looks pretty vague.

For instance, there is a [`cabal-helper`](https://github.com/DanielG/cabal-helper/tree/5ae1c256db3510c513d5d344922f6a1a222a116c) project.
```
>> cabal-head new-build --allow-newer
Resolving dependencies...
cabal-head: Could not resolve dependencies:
next goal: cabal-helper (user goal)
rejecting: cabal-helper-0.8.0.0, cabal-helper-0.7.3.0, cabal-helper-0.7.2.0
(constraint from user target requires ==0.7.1.0)
rejecting: cabal-helper-0.7.1.0 (constraint from user target requires
==0.6.3.1)
rejecting: cabal-helper-0.6.3.1, cabal-helper-0.6.3.0, cabal-helper-0.6.2.0,
cabal-helper-0.6.1.0, cabal-helper-0.6.0.0, cabal-helper-0.5.3.0,
cabal-helper-0.5.1.0, cabal-helper-0.5.0.0, cabal-helper-0.4.0.0,
cabal-helper-0.3.9.0, cabal-helper-0.3.8.0, cabal-helper-0.3.7.0,
cabal-helper-0.3.6.0, cabal-helper-0.3.5.0, cabal-helper-0.3.4.0,
cabal-helper-0.3.3.0, cabal-helper-0.3.2.1, cabal-helper-0.3.2.0,
cabal-helper-0.3.1.0, cabal-helper-0.3.0.0, cabal-helper-0.2.0.0,
cabal-helper-0.1.0.1, cabal-helper-0.7.0.1, cabal-helper-0.5.2.0,
cabal-helper-0.1.0.0 (constraint from user target requires ==0.7.1.0)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: cabal-helper
```

For user it might be unclear, that the reason of such a failure is that all directories with .cabal files are added, but the repo includes a hackage-metadata directory with multiple cabal files for cabal-helper.

Wouldn't it be better to aware user of all the packages that was added to build in this error?