BaseTools: use set instead of list
    
    as we only do membership (in) testing for this, set is better
    
    Cc: Liming Gao <liming.gao@intel.com>
    Cc: Yonghong Zhu <yonghong.zhu@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
    Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>