Move the public API out of oslo.messaging to oslo_messaging. Retain
the ability to import from the old namespace package for backwards
compatibility for this release cycle.
bp/drop-namespace-packages
Co-authored-by: Mehdi Abaakouk <mehdi.abaakouk@enovance.com>
Change-Id: Ia562010c152a214f1c0fed767c82022c7c2c52e7