- Fix zip entry paths. The Go proxy spec requires zip entries to conform
  to `module@version/file`, where `file` is the path within the module.
- Fix /v2+ handling. For major versions 2+, the module name must include
  the major version as a suffix, e.g. /v2.
- Handle case encoding. Requests to the Go proxy encode uppercase
  characters in URLs as '!' followed by the character in lowercase.
- Per Zoom discussion with @trizzi, @sabrams, and team, modules with an
  invalid module name in go.mod will be ignored, initially.