Supporters of the anti-illegal immigration measure adopted in Prince William County last week have argued that its most important purpose is to send a powerful signal to the county's mostly Latino illegal immigrants that they are no longer welcome. It appears the message has already been received: Terrified that new policies will lead to mass deportations, illegal immigrants and the many legal immigrant relatives and friends who live with them have been moving out of Prince William ever since July, when county supervisors first approved the plan's outline.