BaseTools:PackageDocumentTools import lib error occurs.

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>