hist   []Dir    // History of reported Dirs.
	offset int      // Counter for Next.
}

var dirs Dirs

// dirsInit starts the scanning of package directories in GOROOT and GOPATH. Any
// extra paths passed to it are included in the channel.
func dirsInit(extra ...Dir) {
	if buildCtx.GOROOT == "" {
		stdout, err := exec.Command("go", "env", "GOROOT").Output()
		if err != nil {