To many Chicagoans, the Billy Goat Tavern under Michigan Avenue is simply a great spot for a double cheeseburger and a beer.For tourists, Cubs fans and “Saturday Night Live” aficionados, it’s a historic destination.But for the city’s journalists, it’s a sanctuary.When it opened its location below the Realtor Building in 1964, the restaurant was the go-to watering hole for the city’s four largest newspapers, all located minutes away — the Chicago Tribune and Chicago’s American …