Extend core package stats to whole project

Originally posted by @astrofrog somewhere else: At the 2018 coordination meeting, after `@taldcroft`'s talk, we discussed the idea of extending the stats to the whole project (e.g. contributors, depth of contributor base, etc.). This is just a reminder that we should try and encourage someone to do this.

The issue names no files, tests, or entry points. Start by locating the existing core-package stats implementation, then clarify which whole-project contributor metrics are required; done means the agreed project-wide stats are implemented and verifiable.

Written by the indexing model from the issue text.