pkg returns 4 for "nothing to do" (ansible#23007)

* pkg returns 4 for "nothing to do"

We need to handle this because our own checking for whether there's anything to do returns false negatives in certain circumstances.

We need to rename the response `rc`, because that name is reserved by
Ansible to indicate a failure if it is non-zero.

Fixes ansible#22781 and ansible/ansible-modules-extras#932.

* Don't rename `rc` to to `pkg_rc`, and instead override the failure state.

* Drop mention of renamed variable in `pkg5` module.

lib/ansible/modules/packaging/os/pkg5.py

@@ -131,7 +131,10 @@ def ensure(module, state, packages, params):
         response['results'].append(out)
         response['msg'] += err
         response['changed'] = True
-        if rc != 0:
+        if rc == 4:
+            response['changed'] = False
+            response['failed'] = False
+        elif rc != 0:
             module.fail_json(**response)
 
     module.exit_json(**response)