// walk walks the trees in GOROOT and GOPATH.
func (d *Dirs) walk(roots []Dir) {
	for _, root := range roots {
		d.bfsWalkRoot(root)
	}
	close(d.scan)
}

// bfsWalkRoot walks a single directory hierarchy in breadth-first lexical order.
// Each Go source directory it finds is delivered on d.scan.
func (d *Dirs) bfsWalkRoot(root Dir) {
	root.dir = filepath.Clean(root.dir) // because filepath.Join will do it anyway