Publish repositories as Go modules and move the module path to rodent.software

Publish repositories as Go modules and move the module path to rodent.software

The module named a GitHub repository that does not exist, and a module path
under the forge could not be fetched anyway: the go command resolves an import
path by asking it for a go-import tag first, and the forge served only pages.

Answer the <host>/<repository> discovery probe from the catch-all route with the
repository's tag, naming its Git transport and that prefix, so a public
repository is importable with no vanity service in front of the instance. Only
a repository an anonymous reader can read answers: the tag is fetched without
credentials, so a tag for a private repository would confirm its existence to
anyone who guessed the name. The prefix host comes from public_url, never the
request's Host header, for the same reason the clone URL does.

With the path resolvable, rename the module to rodent.software/vole: imports,
the build line in the getting-started guide, and the packaged units and
metadata follow it. docs/go-modules.md explains the lookup, what the instance
must have, and why a repository named after a route cannot claim the prefix.