This CL changes the default module download and module verification mechanisms
to use the Go module mirror and Go checksum database run by Google.
See https://proxy.golang.org/privacy for the services' privacy policy.
(Today, that URL is a redirect to Google's standard privacy policy,
which covers these services as well. If we publish a more specific
privacy policy just for these services, that URL will be updated to
display or redirect to it.)

See 'go help modules' and 'go help modules-auth' for details (added in this CL).

To disable the mirror and checksum database for non-public modules:

	go env -w GONOPROXY=*.private.net,your.com/*
	go env -w GONOSUMDB=*.private.net,your.com/*

(If you are using a private module proxy then you'd only do the second.)

If you run into problems with the behavior of the go command when using
the Go module mirror or the Go checksum database, please file issues at
https://golang.org/issue/new, so that we can address them for the
Go 1.13 release.

For #25530.

This CL also documents GONOPROXY.
Fixes #32056.

Change-Id: I2fde82e071742272b0842efd9580df1a56947fec
Reviewed-on: https://go-review.googlesource.com/c/go/+/178179
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>