BaseTools: Fix the issue to support windows root directory

Use os.path.relpath to get the relative directory instead of directly trim it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18602 6f19259b-4bc3-4df7-8a09-765794883524

BaseTools/Source/Python/Common/MultipleWorkspace.py

@@ -41,7 +41,7 @@ class MultipleWorkspace(object):
     @classmethod
     def convertPackagePath(cls, Ws, Path):
         if str(os.path.normcase (Path)).startswith(Ws):
-            return os.path.join(Ws, Path[len(Ws) + 1:])
+            return os.path.join(Ws, os.path.relpath(Path, Ws))
         return Path
 
     ## setWs()