Also moves the GOPATH env variable guide to
golang.org/wiki/SettingGOPATH.

Fixes #18294.

Change-Id: I88a2ce550df7466f8d2388d86bc8476dcf3c2ad6
Reviewed-on: https://go-review.googlesource.com/34918Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>