spec: clarify when range x does not evaluate x

Fixes issue 4644.

R=r, adonovan
CC=golang-dev
https://codereview.appspot.com/7307083
cmd/go: reject case-insensitive file name, import collisions

To make sure that Go code will work when moved to a
system with a case-insensitive file system, like OS X or Windows,
reject any package built from files with names differing
only in case, and also any package built from imported
dependencies with names differing only in case.

Fixes issue 4773.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7314104