Clean up mirror repos

When we first moved to the monorepo, many of the package managers we used needed a library to be in its own repo, so we set up individual read-only mirror repos which are copied to when there are changes in this repo.

The context has changed, and we don't need them any more. The represent a significant amount of complexity, and things will be simpler if we can do away with them.

- [x] Change go modules to use VCS module paths into common repo (https://github.com/cucumber/common/pull/1608)
- [ ] Update godog to use VCS module paths too (https://github.com/cucumber/godog/issues/389)
- [ ] [Deal with cucumber.ml](https://github.com/cucumber/cucumber.ml/issues/39)
- [ ] remove scripts for generating / updating submodules
- [ ] remove `/bin/linux/amd64/splitsh-lite`
- [ ] archive the go sub-repos (legacy versions will still need them, I think)
- [ ] delete or move other sub-repos to cucumber-attic
- [ ] remove docs about subrepos
- [ ] relative paths instead of rsync and delete rsync scripts/functions