cmd/go/internal/modfetch: cache latest revinfo in Versions func

cmd/go/internal/modfetch: cache latest revinfo in Versions func

The responses have been cached by the web2 package before removed
it in CL 170879. This change add latest revinfo cache in Versions
func.

Fixes #51391

Change-Id: I73597e0a6b4938238e69d85e1cbbaa9007776db3
Reviewed-on: https://go-review.googlesource.com/c/go/+/403335
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Lee Baokun <bk@golangcn.org>
Reviewed-by: David Chase <drchase@google.com>