gosumcheck: basic go.sum checker

Copied from golang.org/x/exp/sumdb/gosumcheck.

For golang/go#31761.

Change-Id: Ia2cf81372b8e2da9b879f3d737de014eb30ea34b
Reviewed-on: https://go-review.googlesource.com/c/mod/+/176639
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
4 files changed