[release-branch.go1] cmd/go: be clear that import loops are bad

[release-branch.go1] cmd/go: be clear that import loops are bad

««« backport f697709b5082
cmd/go: be clear that import loops are bad

There was mail on golang-nuts a few weeks ago
from someone who understood the message perfectly
and knew he had a cyclic dependency but assumed
that Go, like Python or Java, was supposed to handle it.

R=golang-dev, bradfitz, dave
CC=golang-dev
https://golang.org/cl/6488069

»»»