Six months after losing out on its bid to build a resort casino in Boston, Mohegan Sun is now looking for its next casino venture half a world away – in South Korea. The tribe that owns and operates casinos in Uncasville, Conn., and in the Poconos Mountains in Pennsylvania, announced Tuesday it was in exclusive negotiations to open a first-of-its-kind resort casino at the Incheon International Airport in South Korea, some 7,000 miles away from Boston.