WASHINGTON — President Donald Trump describes drugs flowing across the hinterlands from Mexico, a federal prison population laden with criminals who are in the U.S. illegally and a Texas city transformed by a border barrier into a safe place to live. It’s a misleading and in some ways false picture, and one that underpins his extraordinary declaration of a national emergency at the southern border. Trump opened this past week with a rally in El Paso, Texas, and capped it with his emergency declaration Friday, a move taken to free up billions of dollars for a border wall that Congress refused to give him.