ci: several housekeeping changes

[geoknee]

Description
Follow up changes from the new module structure introduced in #62:

• removes go module caching from CI (it was causing an error and even when fixed was not speeding up CI runs)
• updates readme to explain existence of multiple modules in this repository
• updates CI config to check the running go mod tidy in each module doesn't generate any changes
• gitignores go.work.sum. AFAICT this file doesn't do anything for us in the way I anticipate this repo will be used, but it does get updated from time to time in a way that isn't well documented and could cause confusion.

Context

• The existing workflow has errors because it tries to use go.sum from the top level as a cache key.
• It's not 100% clear whether we should check in the go.work file proposal: ref/mod: mention whether go.work files should be checked into VCS golang/go#53502 . However, IMO it is worth it because it makes editors like VSCode understand the repo structure better so that all the usual editor features "just work".

Tests

manual

Open Questions

• I'm not 100% sure that go.work will change when we update dependencies. Probably about 80% sure. It definitely won't. And I am fairly sure go.work.sum won't change be guaranteed to either.
• I'm not convinced we really need caching at this point. On circle, it took 16s to restore a cache, but downloading the dependencies only took 6s.

[mslipper]

If the numbers above are correct let's strip the caching out for now.

[sebastianst]

LGTM thanks for cleaning that up! Found one typo.

Would also be nice to add a short pointer to the CONTRIBUTING.md guide for adding new chains.