PURLDB: Collect and return Go package metadata on demand from a PURL

See
- https://proxy.golang.org/
- https://proxy.golang.org/rsc.io/quote/@v/v1.0.0.info for example
- https://proxy.golang.org/rsc.io/quote/@v/v1.0.0.zip for download
- https://index.golang.org/index for an index
- https://pkg.go.dev for web scraping
- https://github.com/golang/go/issues/36785
- FetchCode has versions https://github.com/nexB/fetchcode/blob/227fba6e1b47a3370f75ed7b4f5c9d114ab43b6c/src/fetchcode/package_versions.py#L295 not yet metadata
- https://github.com/priv-kweihmann/meta-sca/blob/7ef25019b2c098b33c4dbb23fc5e6703b9c88ad2/scripts/go-gen#L119
- Outdated batch
- https://github.com/nexB/purldb/blob/main/minecode/visitors/golang.py
- https://github.com/nexB/purldb/blob/main/minecode/mappers/golang.py
- https://github.com/SoftwareHeritage/swh-lister/blob/bcf30aba90ba6e6e0bf71cf44f76ac3d2763e6a2/swh/lister/golang/lister.py#L40
- https://github.com/librariesio/libraries.io/blob/cfdcd0cf0ef2d28d905fa2f957be206a3c8b282e/app/models/package_manager/go.rb#L66
- https://github.com/jaredpar/golang-mirror-exp
- https://pkg.go.dev/sitemap/