Short-circuit root-ish check for many deps (#5113)

While looking into #5083 I happened to notice that the dashboard felt very sluggish. I profiled with py-spy and discovered that the scheduler was spending 20% of runtime calculaing `sum(map(len, group._dependencies)) < 5`! A quick print statement showed some task groups depended on 25,728 other groups (each of size 1). We can easily skip those.

I originally had this conditional in #4967 but we removed it for simplicity: #4967 (comment); turns out it was relevant after all!

distributed/scheduler.py

@@ -2485,6 +2485,7 @@ def decide_worker(self, ts: TaskState) -> WorkerState:
         if (
             valid_workers is None
             and len(group) > self._total_nthreads * 2
+            and len(group._dependencies) < 5
             and sum(map(len, group._dependencies)) < 5
         ):
             ws: WorkerState = group._last_worker