SACRAMENTO, Calif. — One of the defendants in a massive murder and drug smuggling case involving the Aryan Brotherhood prison gang pleaded guilty Monday in Sacramento federal court, a signal that prosecutors may have gained a potential source of information as they pursue the case.Samuel Keeton, 41, pleaded guilty before U.S. District Judge Kimberly J. Mueller to a count of conspiracy to participate in a racketeering enterprise and conspiracy to distribute methamphetamine and heroin.Keeton had been held without bail at the Sacramento County Main Jail since June 2019 facing charges of conspirac…