DEERFIELD, Ill. — The U.S. Justice Department is accusing Walgreens of filling millions of prescriptions in the last decade and more without a legitimate purpose, including for dangerous amounts of opioids, according to a lawsuit filed this week. Filed in the U.S. District Court for the Northern District of Illinois, the complaint says the drugstore chain’s pharmacists filled controlled substance prescriptions with clear red flags that indicated they were highly likely to be unlawful.