Issue #24225: Within idlelib files, update idlelib module names.

This follows the previous patch that changed idlelib file names.
Class names that matched old module names are not changed.
Change idlelib imports in turtledemo.__main__.

Exception: config-extensions.def.  Previously, extension section
names, file names, and class names had to match.  Changing section
names would create cross-version conflicts in config-extensions.cfg
(user customizations).  Instead map old names to new file names
at point of import in editor.EditorWindow.load_extension.

Patch extensively tested with test_idle, idle_test.htest.py, a custom
import-all test, running IDLE in a console to catch messages,
and testing each menu item.  Based on a patch by Al Sweigart.

Lib/idlelib/codecontext.py

@@ -1,11 +1,11 @@
-"""CodeContext - Extension to display the block context above the edit window
+"""codecontext - Extension to display the block context above the edit window
 
 Once code has scrolled off the top of a window, it can be difficult to
 determine which block you are in.  This extension implements a pane at the top
 of each IDLE edit window which provides block structure hints.  These hints are
 the lines which contain the block opening keywords, e.g. 'if', for the
 enclosing block.  The number of hint lines is determined by the numlines
-variable in the CodeContext section of config-extensions.def. Lines which do
+variable in the codecontext section of config-extensions.def. Lines which do
 not open blocks are not shown in the context hints pane.
 
 """
@@ -13,7 +13,7 @@ import tkinter
 from tkinter.constants import TOP, LEFT, X, W, SUNKEN
 import re
 from sys import maxsize as INFINITY
-from idlelib.configHandler import idleConf
+from idlelib.config import idleConf
 
 BLOCKOPENERS = {"class", "def", "elif", "else", "except", "finally", "for",
                     "if", "try", "while", "with"}