**Unvalidated version in goproxy ParsePackage**
The module version is read straight from the zip directory path and
never checked, so a crafted upload can leave a newline in it;
`EnumeratePackageVersions` then writes each stored version on its own
line for the `@v/list` endpoint, letting a module advertise fabricated
versions to `go` clients. Validated the parsed version with
`semver.IsValid` inside the parser, matching the version checks the
other package parsers already do.
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
