gddo-server: tee requests directly to pkg.go.dev

Change-Id: Ibe8c66efd7771421709695e94a94eb5e945ba5f4
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/274860
Trust: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>