BaseTools: Autogen - change from list to set

BaseTools: Autogen - change from list to set

by changing from list to set(), we can skip all the preprocessing
to prevent duplication and we dont need to convert to a set() later
on for each use

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>