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.

plugins/test/a_module.py

@@ -0,0 +1,33 @@
+# (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
+
+
+from ansible.plugins.loader import action_loader, module_loader
+
+
+def a_module(term):
+    """
+    Example:
+      - 'community.general.ufw' is community.general.a_module
+      - 'community.general.does_not_exist' is not community.general.a_module
+    """
+    for loader in (action_loader, module_loader):
+        data = loader.find_plugin(term)
+        # Ansible 2.9 returns a tuple
+        if isinstance(data, tuple):
+            data = data[0]
+        if data is not None:
+            return True
+    return False
+
+
+class TestModule(object):
+    ''' Ansible jinja2 tests '''
+
+    def tests(self):
+        return {
+            'a_module': a_module,
+        }