oslo: migrate to namespace-less import paths

Oslo project decided to move away from using oslo.* namespace for all their
libraries [1], so we should migrate to new import path.

This patch applies new paths for:
- oslo.config
- oslo.db
- oslo.i18n
- oslo.messaging
- oslo.middleware
- oslo.rootwrap
- oslo.serialization
- oslo.utils

Added hacking check to enforce new import paths for all oslo libraries.

Updated setup.cfg entry points.

We'll cleanup old imports from oslo-incubator modules on demand or
if/when oslo officially deprecates old namespace in one of the next
cycles.

[1]: https://blueprints.launchpad.net/oslo-incubator/+spec/drop-namespace-packages

Depends-On: https://review.openstack.org/#/c/147248/
Depends-On: https://review.openstack.org/#/c/152292/
Depends-On: https://review.openstack.org/#/c/147240/

Closes-Bug: #1409733
Change-Id: If0dce29a0980206ace9866112be529436194d47e
Cherry-picked from openstack/neutron

HACKING.rst

@@ -12,6 +12,7 @@ Neutron Specific Commandments
 - [N320] Validate that LOG messages, except debug ones, have translations
 - [N321] Validate that jsonutils module is used instead of json
 - [N322] Detect common errors with assert_called_once_with
+- [N323] Enforce namespace-less imports for oslo libraries
 
 Creating Unit Tests
 -------------------