Refactor VPPApi

VPPApiDumper() is its own file, preparing for VPPApiApplier() in an
upcoming commit. VPPApi() itself remains read-only. No need for an empty
__init__.py file.

Update vppcfg to use the correct vpp/dumper.py import

vppcfg

@@ -19,7 +19,7 @@ import yaml
 import logging
 from config import Validator
 from vpp.reconciler import Reconciler
-from vpp.vppapi import VPPApiDumper
+from vpp.dumper import VPPApiDumper
 
 try:
     import argparse