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/action_plugins/uncategorized_plugin.py

@@ -0,0 +1,11 @@
+from ansible.runner import return_data
+
+
+class ActionModule (object):
+    def __init__(self, runner):
+        self.runner = runner
+
+    def run(self, conn, tmp, module_name, module_args, inject,
+            complex_args=None, **kwargs):
+        return return_data.ReturnData(conn=conn, comm_ok=True,
+                                      result={"msg": "uncategorized"})