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/doxygengen_spec.py

@@ -13,7 +13,7 @@
 # THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
 # WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 
-from ...basemodel import doxygen
+from plugins.EdkPlugins.basemodel import doxygen
 import os
 try:
     import wx
@@ -21,9 +21,9 @@ try:
 except:
     gInGui = False
 import re
-from ...edk2.model import inf
-from ...edk2.model import dec
-from ...basemodel.message import *
+from plugins.EdkPlugins.edk2.model import inf
+from plugins.EdkPlugins.edk2.model import dec
+from plugins.EdkPlugins.basemodel.message import *
 
 _ignore_dir = ['.svn', '_svn', 'cvs']
 _inf_key_description_mapping_table = {