`cabal list-bin` should not require a target when there is an obvious default.

[AndreasPK]

Invoking cabal list-bin should just pick the obvious default when possible.

If there is a cabal file with a single executable it seems like an obvious choice to report the path for this target even if it's not explicitly specified.

Or in other words, in cases where cabal run just works, I would expect list-bin to also just work. But for some reason they differ in this regard.