Students have clamored for years for a campus-based pub, and now they're getting one.     Renovations are under way in the Marchese Lounge in the lower portion of Cotter Union that will transform it into a small tavern. The lounge was chosen by a student task force that considered other campus sites but rejected them for legal and aesthetic reasons. Maine law prohibits the sale of alcoholic beverages in residence halls and academic buildings.     The pub was expected to open in February.