[FIX] core: ensure odoo.upgrade __path__ contains real directories

mimic what is done for `odoo.addons` __path__.

closes odoo/odoo#121211

X-original-commit: 2a981e6a561f572742202d9c964d3b1f3aecacf9
Signed-off-by: Christophe Simonis <chs@odoo.com>

odoo/modules/module.py

@@ -114,7 +114,7 @@ def initialize_sys_path():
     legacy_upgrade_path = os.path.join(base_path, 'base', 'maintenance', 'migrations')
     for up in (tools.config['upgrade_path'] or legacy_upgrade_path).split(','):
         up = os.path.normcase(os.path.abspath(tools.ustr(up.strip())))
-        if up not in upgrade.__path__:
+        if os.path.isdir(up) and up not in upgrade.__path__:
             upgrade.__path__.append(up)
 
     # create decrecated module alias from odoo.addons.base.maintenance.migrations to odoo.upgrade