gddo-server: add utm_source to pkg.go.dev redirect URLs

This will reliably indicate to pkg.go.dev that a request is due
to the automatic redirect logic.

Additional logic is added for more accurate mapping of
godoc.org -> pkg.go.dev URLs.

Updates golang/go#37099

Change-Id: Iba380a3b5951d50576c17711990d0ba87857b926
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/224659
Reviewed-by: Julie Qiu <julie@golang.org>