Global Constraints for Cabal

[Gershom B]

Hi all. I've got a branch of cabal with a new feature:
https://github.com/gbaz/cabal/tree/constraints-config

This lets you either pass `constraints-file` on the command line or in
the ~/.cabal/config file.

The constraints-file serves as a "default" cabal.config file for
freezes should one not be provided by a particular package.

Thus, a "minimal" platform can ship with a file that includes the
platform constraints. Later, a user can simply download a new file for
a new constraints set -- say lts -- and swap their config to point to
that instead.

This makes both the platform in its new ideally-more-minimal guise and
stackage play nicer together and with cabal, in a way that required
minimal changes to cabal proper.

Ideally this or a relative can land soon to help enable the minimal
platform plans (the work grew out of the discussion on:
https://github.com/haskell/haskell-platform/issues/206)

There are two places where I wasn't quite sure what to do. In
PackageEnvironment.hs, the tryLoadSandboxPackageEnvironmentFile
doesn't check if this flag is set and handle that case. So this is
when we are already in a sandbox and the question is if we should
_also_ respect this flag, just as a sandbox also respects a
cabal.config file directly in the directory. My gut says "don't
respect it in sandboxes" but i'm not sure.

Additionally, the freezePackages function in Freeze.hs doesn't respect
this flag. I'm not sure exactly what the full call chain here is yet,
so haven't worked out the logic.

Anyway, thoughts?

--gershom