Suggest Versions if not found

If we got far enough into the search process that a requested module was not found, we should suggest the next highest or next lowest version that was found up to that point that would satisfy the specification. This would require some additioanl context tracking during some of the module resolution process.

Start by tracing the module resolution process and identify where a requested module is determined to be missing. Review how version constraints are represented and what context is currently retained during resolution. Done means an unresolved request reports the nearest higher or lower version found that satisfies the specification.

索引モデルが issue の本文から書いたものです。