Make non-import packages lazy
Make non-import packages lazy

6659902a731767b3405d68e515c8edcc3af81119 caused a lot of importing a
lot of things in __init__ which makes loading anything, say session,
really slow. The load time for keystoneclient is really critical since
every client uses it. And having a CLI take several seconds to do
nothing is really bad user experience.

This drops the hot cache import time of keystoneclient.session down to
160ms which is about 60ms faster (down from 220ms without this patch)
for me.

Change-Id: I917503ae54c9abcff417f0a0368abb765a847b6e
Partial-Bug: #1431649
Co-Authored-By: Robert Collins <330de7146e8615bb6d55e7079edb657105eda007@hp.com>

klmitch/python-keystoneclient,sdpp/python-keystoneclient,sdpp/python-keystoneclient,magic0704/python-keystoneclient,Mercador/python-keystoneclient,magic0704/python-keystoneclient,klmitch/python-keystoneclient,Mercador/python-keystoneclient