Make trytond add all paths listed in GUIX_TRYTOND_MODULES_PATH to sys.path.

*** a/trytond/modules/__init__.py	1970-01-01 01:00:01.000000000 +0100
--- b/trytond/modules/__init__.py	2021-12-02 22:17:28.014612267 +0100
***************
*** 31,36 ****
--- 31,48 ----
  
  EGG_MODULES = {}
  
+ def __extend_python_path__():
+     tryton_python_path = os.environ.get("GUIX_TRYTOND_MODULES_PATH")
+     if tryton_python_path:
+         paths = [path[:-16]  # remove "/trytond/modules"
+                  for path in tryton_python_path.split(os.pathsep)]
+         sys.path.extend(paths)
+         # ensure new paths are in the pkg_resources WorkingSet
+         import pkg_resources
+         list(map(pkg_resources.working_set.add_entry, paths))
+ 
+ __extend_python_path__()
+ 
  
  def update_egg_modules():
      global EGG_MODULES
l