cmd/go: add notary simulation and GONOVERIFY support

As an experiment to better understand the impact of
having an authoritative source of truth for module hashes
before the real notary is available, this CL adds the basic
notary authorization checks using a partial whitelist of
known go.sum values for popular modules.

In addition to the temporary whitelist, this CL adds code
implementing $GONOVERIFY, a new 'go help modules-auth',
and clearer error messages for verification mismatches.

See #25530 for notary proposal.
Filed #30601 to remove whitelist when notary lands.

Change-Id: Ibcb6ac39c5e60455edf003d8c20af6932aeb7e88
Reviewed-on: https://go-review.googlesource.com/c/go/+/165380


Reviewed-by: Bryan C. Mills <bcmills@google.com>