Add a Singleton metaclass, use it with Display (#48935)

* Add a Singleton class, use it with Display

* update six import

* Move remaining failes to display singleton

* Fix rebase issues

* Singleton improvements

* Add code-smell for 'from __main__ import display'. ci_complete

* s/self/cls/g

* Add docs for no-main-display

* Address linting issues

* Add changelog fragment. ci_complete

* Implement reentrant lock for class instantiation in Singleton

* Add Display singleton porting guide

lib/ansible/plugins/callback/__init__.py

@@ -32,13 +32,10 @@ from ansible.module_utils.common._collections_compat import MutableMapping
 from ansible.parsing.ajson import AnsibleJSONEncoder
 from ansible.plugins import AnsiblePlugin, get_plugin_class
 from ansible.utils.color import stringc
+from ansible.utils.display import Display
 from ansible.vars.clean import strip_internal_keys
 
-try:
-    from __main__ import display as global_display
-except ImportError:
-    from ansible.utils.display import Display
-    global_display = Display()
+global_display = Display()
 
 try:
     from __main__ import cli

lib/ansible/plugins/callback/dense.py

@@ -31,15 +31,13 @@ from ansible.module_utils.six import binary_type, text_type
 from ansible.module_utils.common._collections_compat import MutableMapping, MutableSequence
 from ansible.plugins.callback.default import CallbackModule as CallbackModule_default
 from ansible.utils.color import colorize, hostcolor
-
-try:
-    from __main__ import display
-except ImportError:
-    from ansible.utils.display import Display
-    display = Display()
+from ansible.utils.display import Display
 
 import sys
 
+display = Display()
+
+
 # Design goals:
 #
 #  + On screen there should only be relevant stuff