refactor inventory Loaders for ansible-inventory

* remove support for loading from executable and static files
* instead use ansible-inventory with fallback to backport
* provide private file dir in task manager for cred injection
* durable management of tmp dirs for user scripts

* new 'scm' source choice for scm-type
* update SCM inventory docs to new reality

awx/main/tests/functional/models/test_inventory.py

@@ -12,7 +12,8 @@ class TestSCMUpdateFeatures:
         inv_src = InventorySource(
             scm_project=project,
             source_path='inventory_file',
-            inventory=inventory)
+            inventory=inventory,
+            source='scm')
         with mock.patch.object(inv_src.scm_project, 'update') as mck_update:
             inv_src.save()
             mck_update.assert_called_once_with()