fileutil: update watcher to use fsnotify and polling (#3663)

* fileutil: update watcher to use fsnotify and polling

* raise timeout

* maybe fix

internal/chanutil/merge.go

@@ -0,0 +1,46 @@
+package chanutil
+
+// Merge merges multiple channels together.
+func Merge[T any](ins ...<-chan T) <-chan T {
+	switch len(ins) {
+	case 0:
+		return nil
+	case 1:
+		return ins[0]
+	case 2:
+	default:
+		return Merge(
+			Merge(ins[:len(ins)/2]...),
+			Merge(ins[len(ins)/2:]...),
+		)
+	}
+
+	in1, in2 := ins[0], ins[1]
+	out := make(chan T)
+	go func() {
+		for {
+			if in1 == nil && in2 == nil {
+				close(out)
+				return
+			}
+
+			select {
+			case item, ok := <-in1:
+				if !ok {
+					in1 = nil
+					continue
+				}
+
+				out <- item
+			case item, ok := <-in2:
+				if !ok {
+					in2 = nil
+					continue
+				}
+
+				out <- item
+			}
+		}
+	}()
+	return out
+}