'cabal check' should warn about open lower bounds

[Saizan]

Dependencies of the form "foo > x" don't really make sense given the hackage version policy, since they exclude x but include x.y which is supposed to have a similar api, and if you want something introduced in a minor (or lesser) release you can explicitly mention that.

Also what is often intended with that notation is foo >= (x+1), causing confusion.

I propose that we allow only closed lower bounds, i.e. foo >= x.

[duncan]

Duplicate of #395

[elga]

*  rio bouygues