A friend of Dzhokhar Tsarnaev was indicted today in federal court on charges of heroin trafficking and possessing an illegal handgun — the same gun authorities believe was used to execute MIT police officer Sean Collier, an official told The Herald.Stephen Silva, 21, of Memorial Drive in Cambridge, is charged with possessing a defaced semi-automatic pistol that matches the make and model of the weapon police believe was used to kill Collier.