Support package collections

[jdnavarro]

I couldn't find an issue related to it, so I'm opening this to use it as a reference for commits related to collections. Quoting @dcoutts from Hackathon2015:

package collections in cabal-install and hackage-server

The idea here is to support package collections directly in our infrastructure. Currently package collections like stackage are implemented indirectly by specially crafted cabal.config files. Collections are also to be distributed via hackage, allowing anyone to define and distribute collections.

A specification was posted to the libraries list recently. There is also some prototype code available for cabal-install. The cabal-install code adds solver support and allows for set-like operations on collections. Nothing has been started yet for hackage-server.

This is an important feature since package collections are one of the two big solutions to cabal hell. This is not a huge project. There's probably enough for two people to work on the client side and two on the server side.

[blackheaven]

Talking with @tchoutri and @dcoutts at ZuriHac, it will solves the security issue: haskell/security-advisories#281

@dcoutts agreed to refresh/formalize the collection algebra.