Fix wrongly used CFile::Exists() for directories instead of CDirectory::Exists()
Currently it can cause hardly detectable errors as some VFS don't return true when directory is requested for CFile. One of VFS of such kind is Win32File.
[posix] [vfs] Implement PosixFile class for accessing local files on Posix platforms
[posix] [vfs] Switch to new PosixFile
[vfs] Remove HDFile