cmd/go: use filepath.SplitList when inspecting GOPATH
cmd/go: use filepath.SplitList when inspecting GOPATH

There exists a test case for this condition, but it only runs on unix systems, which neatly dovetails into the code always using ':' as the list separator.

R=adg, iant
CC=golang-dev
https://codereview.appspot.com/7057052

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang