Plug-ins loaded from top-level plug-in directory

PluginLoader._get_paths, as of 391fb98e, was only finding plug-ins that
were in a subdirectory of one of the basedirs (i.e. in a category
directory).  For example, action_plugins/foo.py would never be loaded,
but action_plugins/bar/foo.py would work.

This makes it so that "uncategorized" plug-ins in the top level of a
directory such as action_plugins will be loaded, though plug-ins in a
"category" subdirectory will still be preferred.  For example,
action_plugins/bar/foo.py would be preferred over action_plugins/foo.py.

test/TestRunner.py

@@ -27,6 +27,7 @@ class TestRunner(unittest.TestCase):
     def setUp(self):
         self.user = getpass.getuser()
         self.runner = ansible.runner.Runner(
+            basedir='test/',
             module_name='ping',
             module_path='library/',
             module_args='',
@@ -77,6 +78,12 @@ class TestRunner(unittest.TestCase):
         assert "localhost" in results['contacted']
         return results['contacted']['localhost']
 
+    def test_action_plugins(self):
+        result = self._run("uncategorized_plugin", [])
+        assert result.get("msg") == "uncategorized"
+        result = self._run("categorized_plugin", [])
+        assert result.get("msg") == "categorized"
+
     def test_ping(self):
         result = self._run('ping', [])
         assert "ping" in result