go/build: implement default GOPATH

go/build: implement default GOPATH

Whenever GOPATH is not defined in the environment, use $HOME/go
as its default value. For Windows systems use %USERPROFILE%/go
and $home/go for plan9.

The choice of these environment variables is based on what Docker
currently does. The os/user package is not used to avoid having
a cgo dependency.

Updates #17262. Documentation changes forthcoming.

Change-Id: I6368fbfbc5afda99d6e64c35c1980076fcf45344
Reviewed-on: https://go-review.googlesource.com/32019
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>