Add option for printing reverse dependency tree with wit inspect

[richardxia]

My strawperson proposal would be to add a --reverse flag to wit inspect, which would basically "flip" the arrows so that we can more easily see what depends on a package rather than what a package depends on.

This wouldn't have a major effect on the --dot case, but in the --tree case, because we don't expand subtrees of duplicate dependencies, this would make it much easier to answer the question "What are all the packages that directly depend on foo?"