Remove a wrong assertion
Merge branch 'master' of github.com:OCamlPro/opam
Fix indentation (no semantic changes)
Tentative fix for #449

Use this as an excuse to clean-up this part of the code which has not been visited since a very long time.
Trim the universe before computing brute force heuristics.

Remark: we don't trim it before as it is useful to have a full universe to better report error messages.
Fix duplication of error messages in case of conflicts (see #474)
Add a function to keep only installable packages
In opam list, only display the installable packages.

This fixes #474
Merge branch 'master' into HEAD