William SHIRER once thought he was ''the greatest of foreign correspondents to cover Moscow.'' In Joseph Alsop's eyes, he was a ''fashionable prostitute'' at the service of the Bolsheviks, and in those of the British writer Malcolm Muggeridge, ''the greatest liar of any journalist I have met in fifty years of journalism.'' The flamboyant, controversial newsman Walter Duranty, who headed The New York Times's Moscow bureau from 1922 to 1936 and enjoyed a longer tenure in that post than anyone on record, has long merited a biography. For beyond his questionable probity, he wielded great influence in shaping American attitudes toward the Soviet Union during its fledgling years, and even played a central role in pressuring Franklin D. Roosevelt to open diplomatic relations with Moscow - an event Duranty would describe, with characteristic opportunism, as ''the ten days that steadied the world.''