Changing the -package dependency resolution algorithm

[Edward Z. Yang]

Excerpts from Edward Z. Yang's message of 2014-07-24 15:57:05 +0100:
> - It assumes *-hide-all-packages* at the beginning.  This scheme
>   probably works less well without that: now we need some consistent
>   view of the database to start with.

Actually, thinking about this, this dovetails nicely with the "package
environments" work the IHG is sponsoring.  The idea behind a package
environment is you specify some set of installed package IDs, which
serves as the visible slice of the package database which is used for
compilation.  Then, ghc called without any arguments is simply using
the *default* package environment.

Furthermore, when users install packages, they may or may not decide
to add the package to their global environment, and they can be informed
if the package is inconsistent with a package that already is in their
environment (mismatched dependencies).  A user can also request to
upgrade a package in their environment, and Cabal could calculate how
all the other packages in the environment would need to be upgraded
in order to keep the environment consistent, and run this plan for the
user.

Cheers,
Edward