ExtensionSystem: Remove the IPlugin back pointer to pluginspec

The price of having to loop in two places seems small compared
to cleaner relations between the classes.

There's a new hack in the helpmanager to make sure we aren't
looping to often. The hack wouldn't be needed if the (odd(?))
check there weren't there.

Change-Id: Ifed50213b2de8feedfb45c185808d163c00c19ca
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/extensionsystem/pluginmanager.cpp

@@ -1556,6 +1556,15 @@ void PluginManager::checkForProblematicPlugins()
     d->checkForProblematicPlugins();
 }
 
+/*!
+    Returns the PluginSpec corresponding to \a plugin.
+*/
+
+PluginSpec *PluginManager::specForPlugin(IPlugin *plugin)
+{
+    return findOrDefault(d->pluginSpecs, equal(&PluginSpec::plugin, plugin));
+}
+
 /*!
     \internal
 */