Warn about deprecated packages

[TobyGoodwin]

I just spent a painful afternoon with cabal. Yesterday I added a dependency on http-wget to my project. Somehow or other, it built at that time (otherwise I would have spotted my mistake sooner). But the next day cabal was unhappy. I was able to deduce from the error messages that http-wget was one of the things making it cross. But I had to ask on #haskell for someone to point out that http-wget is deprecated - it says that in the Synopsis. If there could be a machine-readable deprecated flag, such that cabal would have warned me the moment I added the dependency, I would have avoided that painful afternoon.
(Good news: I rewrote the code to use http-conduits, and it's all much happier now.)
As time goes by, there will be more and more packages that are not maintained and will cause grief like this.

[23Skidoo]

A simple workaround is to mark each module in the package as deprecated, like the GHC manual suggests:

module Wibble {-# DEPRECATED "Use Wobble instead" #-} where