IT HAS BEEN part of the perennial uniform of cool kids for quite some time now. It is the jacket of subcultures: bikers, rock 'n' rollers, punk rockers, metalheads, and rebels. And because the fashion world loves subcultures, the motorcycle jacket will always be the jacket of choice for the absolutely stylish. Every model, actress, and musician has her old faithful version to throw on when she feels the need for that elusive cool factor. It should be in your closet for days when you want to channel Fran? Oise Hardy or Marianne Faithful or the Sexy Pistols. Or all three at once-it definitely communicates a super "It" factor.