Store override in the project directory?

I think I put overrides in `~/.multirust` to keep the list of overrides known in a central place and avoid leaving files all over the file system. It's been pointed out though that if this information were stored in the project directory it could be checked into git.

Start by reading how multirust currently records overrides under ~/.multirust; the issue names no file, test, or entry point. Compare the central storage approach with storing the information in the project directory and determine the desired behavior before implementation. Done requires an agreed storage location and clear project-directory semantics.

Written by the indexing model from the issue text.