Add a deprecation warning when using cabal-dev interactively

[creswick]

I think we can begin to retire cabal-dev, with the recent release of cabal-install-1.18, which supports sandboxed builds and corresponding sandboxed ghci sessions.

Cabal-install now also provides a warning if you use cabal sandboxes for a project that has a cabal-dev directory, encouraging you to transition from cabal-dev.

I think we should do something similar, producing a warning when cabal-dev is invoked to indicate that cabal-install's sandboxes should be used instead. Here's one suggestion:

"""
Cabal-dev is deprecated with the release of cabal-1.18.
To create a sandbox with cabal-1.18, run:
$ cabal sandbox init
"""

I think we should endeavor to squelch this message if cabal-dev is /not/ running in an interactive environment, so we don't break any scripts that may rely on it's silence.