Enable autoloading of inventory plugins (#32709)

* Automatically loads and executes an inventory plugin specified by a standard YAML inventory config file containing a `plugin` key at its root.
* Moved inventory PluginLoader to a shared global instance.

lib/ansible/plugins/loader.py

@@ -582,3 +582,10 @@ netconf_loader = PluginLoader(
     'netconf_plugins',
     required_base_class='NetconfBase'
 )
+
+inventory_loader = PluginLoader(
+    'InventoryModule',
+    'ansible.plugins.inventory',
+    C.DEFAULT_INVENTORY_PLUGIN_PATH,
+    'inventory_plugins'
+)