RIO BRAVO — Empty cocaine packages littered the tall grass like candy wrappers along the bank of the Rio Grande in El Cenizo, a small but growing city 15 miles southeast of Laredo. On closer inspection, each of the 30 or so packages revealed layers — foil, plastic kitchen wrap and brown packing tape. Two large flour sacks were discarded nearby, each with a car seatbelt ingeniously tied diagonally across it for no-hassle transporting. The drugs were long gone, their wrappings not just evidence of a successful crime but another artifact in an outdoor museum of discarded clothes, inner tubes...