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

docs/docsite/rst/dev_guide/developing_plugins.rst

@@ -236,12 +236,9 @@ Here's a simple lookup plugin implementation --- this lookup returns the content
   """
   from ansible.errors import AnsibleError, AnsibleParserError
   from ansible.plugins.lookup import LookupBase
+  from ansible.utils.display import Display
 
-  try:
-      from __main__ import display
-  except ImportError:
-      from ansible.utils.display import Display
-      display = Display()
+  display = Display()
 
 
   class LookupModule(LookupBase):

docs/docsite/rst/dev_guide/testing/sanity/no-main-display.rst

@@ -0,0 +1,12 @@
+Sanity Tests » no-main-display
+==============================
+
+As of Ansible 2.8, ``Display`` should no longer be imported from ``__main__``.
+
+``Display`` is now a singleton and should be utilized like the following::
+
+   from ansible.utils.display import Display
+   display = Display()
+
+There is no longer a need to attempt ``from __main__ import display`` inside
+a ``try/except`` block.

docs/docsite/rst/porting_guides/porting_guide_2.0.rst

@@ -332,8 +332,8 @@ As a simple example we are going to make a hybrid ``fileglob`` lookup plugin.
         from ansible.utils import (listify_lookup_plugin_terms, path_dwim, warning)
     except ImportError:
         # ansible-2.0
-        from __main__ import display
-        warning = display.warning
+        from ansible.utils.display import Display
+        warning = Display().warning
 
     class LookupModule(LookupBase):
 

docs/docsite/rst/porting_guides/porting_guide_2.8.rst

@@ -124,7 +124,28 @@ Plugins
 Porting custom scripts
 ======================
 
-No notable changes.
+Display class
+-------------
+
+As of Ansible 2.8, the ``Display`` class is now a "singleton". Instead of using ``__main__.display`` each file should
+import and instantiate ``ansible.utils.display.Display`` on it's own.
+
+**OLD** In Ansible 2.7 (and earlier) the following was used to access the ``display`` object:
+
+.. code-block:: python
+
+   try:
+       from __main__ import display
+   except ImportError:
+       from ansible.utils.display import Display
+       display = Display()
+
+**NEW** In Ansible 2.8 the following should be used:
+
+.. code-block:: python
+
+   from ansible.utils.display import Display
+   display = Display()
 
 Networking
 ==========