// closed when the dir's package is installed.
var installed = make(map[string]chan struct{})
var installedMu sync.Mutex

func install(dir string) {
	<-startInstall(dir)
}

func startInstall(dir string) chan struct{} {
	installedMu.Lock()
	ch := installed[dir]
	if ch == nil {
		ch = make(chan struct{})
		installed[dir] = ch
		go runInstall(dir, ch)
	}