It's similar to what cmd/go/internal/web package does
when cfg.BuildV is set. The web2 package is what
cmd/go/internal/modfetch uses, so this change allows us
to trace web requests go get command in module mode
sends for modfetch.

Change-Id: If387efd8a8698c816bf267d1e6c6766fd357c298
Reviewed-on: https://go-review.googlesource.com/c/153640
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>