~/.opam/repo/index is now a map between *full* package to repo names

It used to be a map between package names to repo names, and this was very costly to know which repository to look for for a given package
Installed packages without any repository are not available
Speed-up 'opam list'
Always display the repository a package comes from with 'opam info'
Improve the error message in case of corrupted ~/.opam/repo/index