Hoist the notifier to its own module

The notifier module needs to be hoisted out of the misc utility
file so that it can be depended on existing by users in a well
defined (non-utility) location.

This change does this hoisting process & creates a new module
and places the existing code there, then creates a deprecated
proxy that exists at the old location (this will be removed
in the next version + 1).

In a future change (in 0.5) we can remove this old location and
remove all references to the previous location (until then we
must keep the old location being used to ensure subclass checks
and other types checks function properly).

Part of blueprint top-level-types

Change-Id: I47fac110adf7cec5c859c2e055c1ceb1f25a7fbd
Merge "Hoist the notifier to its own module"