With Navy leaders envisioning a fleet of smaller, faster and more agile ships, the future of the country’s large shipbuilders is sinking fast. Last month, Navy leaders pulled the plug on a plan to subsidize the United States’ two large shipyards — Maine’s Bath Iron Works and Northrop Grumman’s Ingalls shipyard in Pascagoula, Miss. — through alternating construction of a new class of surface destroyers at each yard.