cabal install / cabal update should give warning when preferred versions will result in something other than the latest version on hackage

(Imported from [Trac #408](http://hackage.haskell.org/trac/hackage/ticket/408), reported by tphyahoo on 2008-11-16)

The natural assumption, unless you know better, is that cabal install -- and especially cabal upgrade -- will install the latest verion of a package on hackage. But the preferred-version machinery invalidates this assumptijon.

The suggestions is to emit a warning like

  cabal install haxml

  Downloading HaXml-1.13.3...

  Note: the preferred version is HaXml-1.13.3 but latest version is

  HaXml-1.19, so put

  Build-Depends: HaXml == 1.19.4

  in your cabal file if you want that version.

I would have this message pop up for both cabal install and cabal upgrade

See [http://groups.google.com/group/fa.haskell/browse_thread/thread/c39dca259a9489ef?q=cabal+preferred+version](http://groups.google.com/group/fa.haskell/browse_thread/thread/c39dca259a9489ef?q=cabal+preferred+version) for discussion thread.