Add test plugin which allows to query whether a module or action plugin is available (#3628)

* Add test which allows to query whether an action is available.

* Add documentation.

* Fix FreeBSD / macOS compatibility.

* Rename an_action -> a_module.

* Forgot one place.

tests/integration/targets/test_a_module/library/local_module.py

@@ -0,0 +1,32 @@
+#!/usr/bin/python
+# -*- coding: utf-8 -*-
+
+# (c) 2021, Felix Fontein <felix@fontein.de>
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+DOCUMENTATION = '''
+---
+module: local_module
+short_description: Test local module
+description:
+  - This is a test module locally next to a playbook.
+author: "Felix Fontein (@felixfontein)"
+options: {}
+'''
+
+EXAMPLES = ''' # '''
+
+RETURN = ''' # '''
+
+from ansible.module_utils.basic import AnsibleModule
+
+
+def main():
+    AnsibleModule(argument_spec={}).exit_json()
+
+
+if __name__ == '__main__':
+    main()