Introduce wireit 'explain'

Sometimes tracking why a particular command in a monorepo was run takes a while and hinges a lot on understanding deeply how Wireit works, which might not always be the case for everyone working with the codebase. Being able to run `wireit explain ` similar to `tsc --explainFiles` to see why a particular dependent command is included/cached would be helpful. It would make it a lot easier to refine `files`/`outputs` passed to Wireit.

I imagine it would by a dry-run type of command, since we often don't want to modify the cache/files we're trying to observe. This would be run directly rather than through `npm run`.