In the latest step in its quest to become a supplier of modular interiors, Lear Corp. will buy Masland Corp. for about $385 million. Masland, which produces automotive carpeting and luggage compartment trim, will give a new dimension to Lear's product line, which expanded from seating systems to interior doors and trim last summer when it acquired Automotive Industries Inc. Masland, which employs 3,200 workers in 15 plants spread across four countries, earned $47 million on sales of $497 million in 1995. Lear Chairman Kenneth L. Way frequently talks about manufacturing interior modules that include everything from the instrument panel to seats to door trim and even electrical wiring that Lear would deliver as an integrated unit into customers' assembly plants.