WASHINGTON – The Justice Department is investigating allegations of sexual assault against former Congressman Eric Swalwell, a source familiar with the matter told The Post Thursday. Prosecutors in New York and detectives in Los Angeles are already scrutinizing Swalwell (D-Calif.), who has been accused by at least five women of sexual misconduct , including at least two instances of rape. Swalwell has denied criminal wrongdoing, but copped to “mistakes in judgment” in statements ending his