rabbit_plugins: Add which_plugin/1 to query which plugin provides a module

[Why]
This will be used in a later commit to find the auth backend plugin that
provides a configured auth backend module.

[How]
We go through the list of available plugins, regardless if they are
enabled or not, then look up the given module in the list of modules
associated with each plugin's application.

(cherry picked from commit a8bef77)

Author: dumbbell

deps/rabbit/src/rabbit_plugins.erl

@@ -18,6 +18,7 @@
 -export([is_strictly_plugin/1, strictly_plugins/2, strictly_plugins/1]).
 -export([plugins_dir/0, plugin_names/1, plugins_expand_dir/0, enabled_plugins_file/0]).
 -export([is_enabled/1, is_enabled_on_node/2]).
+-export([which_plugin/1]).
 
 % Export for testing purpose.
 -export([is_version_supported/2, validate_plugins/2]).
@@ -290,6 +291,56 @@ running_plugins() ->
     ActivePlugins = active(),
     {ok, [{App, Vsn} || {App, _ , Vsn} <- rabbit_misc:which_applications(), lists:member(App, ActivePlugins)]}.
 
+-spec which_plugin(Module) -> Ret when
+      Module :: module(),
+      Ret :: {ok, PluginName} | {error, Reason},
+      PluginName :: atom(),
+      Reason :: no_provider.
+%% @doc Returns the name of the plugin that provides the given module.
+%%
+%% If no plugin provides the module, `{error, no_provider}' is returned.
+%%
+%% The returned plugin might not be enabled, thus using the given module might
+%% not work until the plugin is enabled.
+%%
+%% @returns An `{ok, PluginName}' tuple with the name of the plugin providing
+%% the module, or `{error, no_provider}'.
+
+which_plugin(Module) ->
+    Plugins = list(),
+    which_plugin(Plugins, Module).
+
+which_plugin([#plugin{name = Name} | Rest], Module) ->
+    %% Get the list of modules belonging to this plugin.
+    ModulesKey = case application:get_key(Name, modules) of
+                     {ok, _} = Ret ->
+                         Ret;
+                     undefined ->
+                         %% The plugin application might not be loaded. Load
+                         %% it temporarily and try again.
+                         case application:load(Name) of
+                             ok ->
+                                 Ret = application:get_key(Name, modules),
+                                 _ = application:unload(Name),
+                                 Ret;
+                             {error, _Reason} ->
+                                 undefined
+                         end
+                 end,
+    case ModulesKey of
+        {ok, Modules} ->
+            case lists:member(Module, Modules) of
+                true ->
+                    {ok, Name};
+                false ->
+                    which_plugin(Rest, Module)
+            end;
+        undefined ->
+            which_plugin(Rest, Module)
+    end;
+which_plugin([], _Module) ->
+    {error, no_provider}.
+
 %%----------------------------------------------------------------------------
 
 prepare_plugins(Enabled) ->