Put the sumdb cache in the usual pkg/mod/cache/download dir, rather
than the new pkg/mod/download/cache dir which I presume was a typo.

Change-Id: Id162f24db30f509353178ca0c8cc7a4dabc927e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/183318
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>