SAN DIEGO — In the vast migrant camp that sprung up this week on a patch of U.S. soil between Tijuana and San Diego, a striking system of order has emerged, even as anxiety and uncertainty swell. The Africans in the camp — from Ghana, Somalia, Kenya, Guinea, Nigeria — have one leader, a tall Somali man, who communicates with aid groups about how many blankets, diapers and sanitary pads they need that day.