BaseTools:PackageDocumentTools import lib error occurs.

Steps:
 1. Download edk2 tree
 2. Build BaseTools
 3. Go to edk2\BaseTools\Scripts\PackageDocumentTools
    to run packagedoc_cli.py

An error occurs if relative imports are used when running
a file alone

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/Scripts/PackageDocumentTools/plugins/EdkPlugins/edk2/model/dsc.py

@@ -11,9 +11,9 @@
 # WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 #
 
-from ...basemodel import ini
+from plugins.EdkPlugins.basemodel import ini
 import re, os
-from ...basemodel.message import *
+from plugins.EdkPlugins.basemodel.message import *
 
 class DSCFile(ini.BaseINIFile):
     def GetSectionInstance(self, parent, name, isCombined=False):