Fix import bug that prevents pickling of norm functions

https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly
https://stackoverflow.com/questions/43819653/pickling-objects-imported-with-importlib-util/53696167#53696167

map2map/utils/imp.py

@@ -1,4 +1,5 @@
 import os
+import sys
 import importlib
 
 
@@ -51,6 +52,7 @@ def import_attr(name, *pkgs, callback_at=None):
 
         spec = importlib.util.spec_from_file_location(mod, callback_at)
         module = importlib.util.module_from_spec(spec)
+        sys.modules[mod] = module
         spec.loader.exec_module(module)
 
         return getattr(module, attr)