GeezerVison

Refs #306, builds on #428.

Registers golang for storage.retention.

Package rules use the module path as in go.mod, e.g. pkg:golang/github.com/stretchr/testify. The proxy stores module paths lowercased, so a rule matches whatever case it's written in. A major version (/v2, or .v3 on gopkg.in) is a module of its own and needs its own rule. Keys in the !-escaped form from proxy URLs and the module cache (github.com/!azure/...) are refused at startup, since they could never match.

Because paths are lowercased, two modules that differ only in case already share one cache entry on main. That isn't changed here.