DY: Without being really Ma’amik into this Inyan, suffice it to say that the Mechaber is pretty B’feirush like Yitay said. These Poskim were clearly adding “Tznifim Lehachmir” because they were trying to fix a Geder that was being trampled upon. Also, even if pants stayed a Kli Gever back then it’s nearly impossible to say they are one now.
(See SHU”T HaRashba 4:90, by the way, where anyone who studies history can tell you should hold that pants are an Issur D’oraisa of Beged Isha for men to wear and dresses (which are very similar to tunics) could be an Issur D’oraisa of Begged Ish for a woman to wear.)