re-introduce linecache usage again

This removes the if/else case when the file_name is not present, and
relies once again on linecache, fixing a problem where `source` would
always be set to None in remote modules

Author: alfredodeza

execnet/gateway.py

@@ -4,6 +4,7 @@
 (c) 2004-2013, Holger Krekel and others
 """
 import inspect
+import linecache
 import os
 import sys
 import textwrap
@@ -111,10 +112,8 @@ def remote_exec(self, source, **kwargs):
         file_name = None
         if isinstance(source, types.ModuleType):
             file_name = inspect.getsourcefile(source)
-            if not file_name:
-                source = inspect.getsource(source)
-            else:
-                source = None
+            linecache.updatecache(file_name)
+            source = inspect.getsource(source)
         elif isinstance(source, types.FunctionType):
             call_name = source.__name__
             file_name = inspect.getsourcefile(source)