Improve reporting of dependency resolution failures

[jheiss]

We added some checking for the availability of the dependencies of the package that the user is trying to install. If any of those are unavailable we give the user a message like "Unable to find any packages which satisfy ". However, that checking only goes one level deep. If all versions of the package depend on some package which in turn has dependencies we can't satisfy the user currently just gets the "checked 10000 solutions" error, which is very non-specific and confusing. Somehow we should give the user more specific feedback.

Note that this is somewhat difficult because various versions of packages and dependencies that we check can be unsatisfiable for different reasons and reporting on all of those could be overwhelming. Perhaps we should just reporting on any dependency failures for the most recent version of each candidate package?

Was: https://sourceforge.net/apps/trac/tpkg/ticket/26

[jheiss]

Some specific, trivial examples. Although this ticket is really about a deeper, more complex issue around packages with dependencies these are symptoms of the same problem:

In this first case the message ought to at least reflect more specifics of what the user asked for (i.e. "which satisfy ruby >= 1.9" instead of just "which satisfy ruby").

% tpkg -i ruby>=1.9
Unable to find any packages which satisfy ruby
tpkg.rb:3491:in `check_requests': Unable to satisfy the request(s). Try running with --debug for more info (RuntimeError)

That's not specific to inequality BTW:

% tpkg -i ruby=1.9
Unable to find any packages which satisfy ruby

Was: https://sourceforge.net/apps/trac/tpkg/ticket/26#comment:1