Backport PR #9936: added namespace condition in IPcompleter

added

```
if self.use_main_ns:
           self.namespace = __main__.__dict__
```
in IPcompelter.py as suggested by Carreau sir,  in  #9897

IPython/core/completer.py

@@ -1164,6 +1164,9 b' class IPCompleter(Completer):'
         if cursor_pos is None:
             cursor_pos = len(line_buffer) if text is None else len(text)
 
+        if self.use_main_ns:
+            self.namespace = __main__.__dict__
+
         if PY3:
 
             base_text = text if not line_buffer else line_buffer[:cursor_pos]