cmd/go: add GODEBUG tokens for debugging lazy module loading

GODEBUG=lazymod=log causes the go command to log a stack dump whenever
the full module graph is loaded in a lazy module.

GODEBUG=lazymod=strict does the same, but also terminates the command
with a nonzero exit code.

For #36460

Change-Id: Ia5a4c46069044bcc157b285f64c2392990d70bd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/315411


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>