Add a ghc -show-packages mode to display ghc's view of the package env

You can use ghc -show-packages, in addition to any -package -package-conf
-hide-package, etc flags and see just what ghc's package info looks like.
The format is much like ghc-pkg show.

Like the existing verbose tracing, but a specific mode.
Re-introduce pretty printed package info (Cabal handled this previously).