[FIX] core: support controllers outside addons

With the iot-box it is possible to download modules from an odoo server
into the iot-box. Those odoo modules are `exec`-like and exposed on the
box. With those modules come some controllers. Because they were
`exec`-like, they lack a python module name which makes
`_generate_routing_rules` to reject them.

With this fix, we authorise to declare controllers outside of odoo
addons. Those controllers cannot be extended, nor can they override
other controllers.

closes odoo/odoo#86275

Signed-off-by: Julien Castiaux <juc@odoo.com>