cmd/go: adjust module-related logging

Suppress “finding” messages unless they are unusually slow, and
“extracting” messages always (they almost always occur conjunction
with “downloading”, which is already logged).

Log “found” messages for module dependencies added to satisfy missing
import paths.

Log top-level version changes in 'go get' when the selected version
is not identical to the version requested on the command line.

Updates #26152
Updates #33284

Change-Id: I4d0de60fab58d7cc7df8a2aff05c8b5b2220e626
Reviewed-on: https://go-review.googlesource.com/c/go/+/204777


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>