Fixed bogus warning when using internal solver
Compute Cudf versions for available packages only

This avoids unneeded work and gives better conflict messages (we could previously
have a constraint referring to a not-available package, and the version would
then only be printed as '??'. Now it's mapped to available versions.
Print meaningful messages when a package is not available.

Closes #595
Merge pull request #1238 from OCamlPro/fix-conflicts-messages

Smaller fixes for conflict and error messages