BaseTools: Workspace/MetaFileParser - refactor dicts

BaseTools: Workspace/MetaFileParser - refactor dicts

make defaultdict to avoid initialize inner items
to empty the dict, call clear instead of making a new object

v2 - to empty the dict, dont re-run constructor, just call .clear()
in post process API also.

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>