cmd/go: describe dependencies in build list error messages

mvs.BuildList reports errors with a chain of modules to make it
clear why the module where the error occurred was part of the
build. This is a little confusing with "go get -u" since there are
edges in the module graph for requirements and for updates.

With this change, we now print "requires" or "updates to" between
each module version in the chain.

Updates #30661

Change-Id: Ie689500ea86857e715b250b9e0cae0bc6686dc32
Reviewed-on: https://go-review.googlesource.com/c/go/+/171150


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