[Inventory] Cache the result of enumerating groups and host names

for `VariableManager._get_magic_variables()`.

This saves a lot of time re-iterating the nearly always constant global
list of groups and their members.

Generate once and cache, and invalidate cache in case `add_host:` or
`group_by:` are used.

test/units/plugins/strategies/test_strategy_base.py

@@ -237,6 +237,7 @@ class TestStrategyBase(unittest.TestCase):
         mock_inventory.get_host.side_effect = _get_host
         mock_inventory.get_group.side_effect = _get_group
         mock_inventory.clear_pattern_cache.return_value = None
+        mock_inventory.clear_group_dict_cache.return_value = None
         mock_inventory.get_host_vars.return_value = {}
 
         mock_var_mgr = MagicMock()