goinstall: support GOPATH; building and installing outside the Go tree

goinstall: support GOPATH; building and installing outside the Go tree

For example, with GOPATH set like so
        GOPATH=/home/adg/gocode
And after creating some subdirectories
        mkdir /home/adg/gocode/{bin,pkg,src}

I can use goinstall to install the github.com/nf/goto web server,
which depends on the github.com/nf/stat package, with
        goinstall github.com/nf/goto

This downloads and installs all dependencies (that aren't already
installed) like so
        /home/adg/gocode/bin/goto
        /home/adg/gocode/pkg/darwin_amd64/github.com/nf/stat.a
        /home/adg/gocode/src/github.com/nf/goto/...
        /home/adg/gocode/src/github.com/nf/stat/...

R=rsc, niemeyer
CC=golang-dev
https://golang.org/cl/4438043