solution with package "sync"

exercise-web-crawler.go

// https://tour.go-zh.org/concurrency/10
func Crawl(url string, depth int, fetcher Fetcher) {
	var wg sync.WaitGroup
	var mux sync.Mutex
	fetched := make(map[string]bool)
    	var crawl func(url string, depth int, fetcher Fetcher)
	crawl = func (url string, depth int, fetcher Fetcher) {
		defer wg.Done() // ref - 1
		if depth <= 0 {
			return
		}
		mux.Lock() // mutex
		if !fetched[url] {
			fetched[url] = true // set flag
			mux.Unlock()
		} else { // fetch already started or done
			mux.Unlock()
			return
		}
		body, urls, err := fetcher.Fetch(url) // start fetch
		if err != nil {
			fmt.Println(err)
			return
		}
		fmt.Printf("found: %s %q\n", url, body)
		for _, u := range urls {
			wg.Add(1) // ref + 1
			url := u // necessary?
			go crawl(url, depth-1, fetcher)
		}
	}
	wg.Add(1)
	crawl(url, depth, fetcher)
	wg.Wait()
}