[REF] packaging: make odoo a namespaced package

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.

odoo-bin

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
+__import__('pkg_resources').declare_namespace('odoo.addons')
 import odoo
 
 if __name__ == "__main__":
     odoo.cli.main()
-