Unexpected Formation of 2-Phenyloxazolo[3,2-a]benzimidazole from 2-Chloro-1-phenacylbenzimidazole and an Example of Its Transformation into 3-Phenyl-1,4-dihydro-1,2,4-triazino[4,3-a]benzimidazole by the Action of Hydrazine Hydrate