Deprecates the previous pkg_resources-style packages. Our use was
not entirely spec compliant, and it broke with setuptools 31.
More information in https://github.com/odoo/odoo/pull/15718

Instead, use the pkgutil-style namespace packages.
This is spec compliant as long as all other distributions providing
odoo.addons use PEP 420 namespace packages (i.e. no __init__.py).
So this mechanism fully works on Python 3 only, but does not break
anything on Python 2.

See also:
https://packaging.python.org/guides/packaging-namespace-packages/#pkgutil-style-namespace-packages

Closes #19517
Fixes #15718