Change index representation to avoid pure exceptions

That is, change cabal-install's internal index type to something like `PackageId -> Either Problem Content`. This will however have consequences all through the code base. (And note that until we have author signing, we haven't actually introduced any new kinds of errors that can occur while accessing the index.)

Start by locating cabal-install's internal index type and tracing every use across the code base. Compare the current exception-based access with the proposed Either Problem Content representation; done means all affected uses consistently handle the result, with no further scope or tests specified in the issue.

Written by the indexing model from the issue text.