Work around trytond.module not being a real namespace module.

Solution is to add all trytond module's locations to
trytond.modules._path__. This will make trytond.module behave much
like a namespace module.
Adding to __path__ is done in update_egg_modules() to ensure __path__
is updated whenever the list of egg modules is updated.

*** a/trytond/modules/__init__.py	1970-01-01 01:00:01.000000000 +0100
--- b/trytond/modules/__init__.py	2021-12-02 18:12:15.385101986 +0100
***************
*** 38,43 ****
--- 38,46 ----
          import pkg_resources
          for ep in pkg_resources.iter_entry_points('trytond.modules'):
              EGG_MODULES[ep.name] = ep
+             path = os.path.join(ep.dist.location, 'trytond', 'modules')
+             if not path in __path__ and os.path.isdir(path):
+                 __path__.append(path)
      except ImportError:
          pass