Start odoo-bin using a tier module (like `python3 -m pdb odoo-bin`), the
odoo/addons __path__ contains the relative link to the modules instead
of the absolute link like when directly started (`python3 odoo-bin`).

This cause trouble importing modules because it reuses the relative path
from within other directories.

closes odoo/odoo#39182

X-original-commit: 941e051fc6b33e7a90400c77d63d589049b027a6
Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>

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

by auto-extending addons path with odoo.addons.__path__.

This patch also introduce
  * an explicit declaration of odoo.addons as a namespace package
    This is necessary because the standard way of declaring namespace
    packages in setup.py does not work as long as odoo/__init__.py
    contains code.
  * a more reliable way to find odoo root path.