fix: always print plugins sorted by name

`tutor plugins list` used to print plugins in random oredr. To be honest
this has always bothered me.

CHANGELOG.md

@@ -4,6 +4,7 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Improvement] Make `tutor plugins list` print plugins sorted by name.
 - [Improvement] Ignore Python plugins which cannot be loaded.
 
 ## v12.1.6 (2021-11-02)

tutor/plugins.py

@@ -363,11 +363,14 @@ def iter_installed(cls) -> Iterator[BasePlugin]:
         prevent too many re-computations, which happens a lot.
         """
         installed_plugin_names = set()
+        plugins = []
         for PluginClass in cls.PLUGIN_CLASSES:
             for plugin in PluginClass.iter_installed():
                 if plugin.name not in installed_plugin_names:
                     installed_plugin_names.add(plugin.name)
-                    yield plugin
+                    plugins.append(plugin)
+        plugins = sorted(plugins, key=lambda plugin: plugin.name)
+        yield from plugins
 
     def iter_enabled(self) -> Iterator[BasePlugin]:
         for plugin in self.iter_installed():