code.html: update to reflect that package names need not be unique
code.html: update to reflect that package names need not be unique

Fixes issue 1507.

R=golang-dev, ehog.hedge, rsc
CC=golang-dev
http://codereview.appspot.com/4160049

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go