gddo-server: fetch path on pkg.go.dev 404

When a godoc.org requests results in a 200, but a 404 on pkg.go.dev,
make a fetch request to pkg.go.dev to fetch that path and log the
output.

Change-Id: I1ca12732317d7e90b42eff022357067d21f84f9b
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/279038
Trust: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>