The former Justice Department official who led the agency’s investigation of Libor and the financial crisis is taking a post at a law firm that represents Wall Street firms who have faced federal scrutiny. Lanny Breuer left as head of the Justice Department’s criminal division last month. In the latest sign of the revolving door between Wall Street and agencies that purport to oversee it, Breuer is returning to his former employer, the law firm Covington & Burling, where he is expected to defend corporate clients and receive a salary of $4 million a year.