ExtensionManager: Prioritize plugin metadata data from service

This change enables the Qt Creator Extension service data to override
data of locally installed plugins (so far it was the other way around).
We might want to add more description text, links, images or tags to
plugins. And we want to be able do that independently of Qt Creator
releases.

This change also allows for substantial simplification of the "data
merging" code.

The parsing of extension plugin dependencies from the json data needed
to be fixed. See dependenciesFromJson()

Change-Id: Ia0433f0e0c7a0f13c43e0569c0915b7d08f7370a
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/plugins/extensionmanager/extensionsbrowser.cpp

@@ -381,7 +381,7 @@ void ExtensionsBrowser::fetchExtensions()
     const auto onQueryDone = [this](const NetworkQuery &query, DoneWith result) {
         if (result != DoneWith::Success) {
 #ifdef WITH_TESTS
-            // Available test sets: "defaultpacks", "varieddata", "thirdpartyplugins"
+            // Available: "augmentedplugindata", "defaultpacks", "varieddata", "thirdpartyplugins"
             d->model->setExtensionsJson(testData("defaultpacks"));
 #endif // WITH_TESTS
             return;