Register imported CLI sub-namespaces in sys.modules (IronLanguages#1991)

* Register imported CLI sub-namespaces in sys.modules

* Add tests

Author: BCSharp

src/core/IronPython/Runtime/Importer.cs

@@ -127,7 +127,11 @@ private static object ImportModuleFrom(CodeContext/*!*/ context, object from, Ar
             string name = parts.Array[parts.Offset + parts.Count - 1];
             if (from is NamespaceTracker ns) {
                 if (ns.TryGetValue(name, out object val)) {
-                    return MemberTrackerToPython(context, val);
+                    object ret = MemberTrackerToPython(context, val);
+                    if (ret != null && val is NamespaceTracker retns && !context.LanguageContext.SystemStateModules.ContainsKey(retns.Name)) {
+                        context.LanguageContext.SystemStateModules[retns.Name] = ret;
+                    }
+                    return ret;
                 }
             }
 

tests/suite/test_cliclass.py

@@ -1199,11 +1199,14 @@ def test_serialization(self):
                 System.StringSplitOptions.RemoveEmptyEntries,
                 ]
 
-        if False:
-            # TODO: Enum types are not picklable if defined in nested namespaces
-            # https://github.com/IronLanguages/ironpython3/issues/1989
-            clr.AddReference("System.Text.Json")
-            data.append(System.Text.Json.JsonValueKind.Object) # byte-based enum
+        if is_cli:
+            # enum types
+            clr.AddReference("Microsoft.Scripting")
+            import Microsoft.Scripting
+            data.append(Microsoft.Scripting.Severity.Warning)  # different namespace than System
+            if is_netcoreapp:
+                clr.AddReference("System.Text.Json")
+                data.append(System.Text.Json.JsonValueKind.Object) # byte-based enum
 
         data.append(list(data))     # list of all the data..
         data.append(tuple(data))    # tuple of all the data...