SCUMM: Always treat the Steam versions as Windows internally

The Steam versions share the same DOS data files with different
game executables. Thus, we always treat them as Windows versions
internally, so that the code used for the classic Mac versions
isn't used for these versions. For game detection purposes, the
game platform is still shown as Mac, with a game variant of "Steam"
SCUMM: Only handle index files inside ScummSteamFile::open()