Reactions of 6‐Aryl‐5‐benzoyl‐4‐dichloromethyl‐4‐hydroxyhexahydropyrimidin‐2‐ones with Hydrazine Hydrate: A New Simple and Efficient Route to 4‐Aryl‐5‐phenyl‐3,4‐dihydropyrimido[4,5‐d]pyridazin‐2(1H)‐ones.