gddo-server: change redirect for github.com/golang/go requests

Requests to github.com/golang/go and github.com/golang/go/src are now
redirected to pkg.go.dev/std.

Requests to github.com/golang/go/src/<path> are redirected to
pkg.go.dev/<path>.

Change-Id: Iaf9f52d17fc19c3e6c325c260de3f070da32a21b
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/279036
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
2 files changed