MP Lobby: fixed being prompted to download non-required addons when j…

…oining a game.

Fixes #2476.

It turns out I had implemented 09f919e incorrectly.
All required addons, including those needed for modifications, were already added
to the game's config by the host before being sent to the server. There was no need
to parse the active list of modifications separately. I appear to have confused the
idea of addon type vs the modification itself and assumed one always equaled to the
other. Which is wrong. For example, you may have multiple mods active from the same
addon.

src/game_initialization/lobby_data.cpp

@@ -224,18 +224,19 @@ game_info::game_info(const config& game, const config& game_config, const std::v
 	, required_addons()
 	, addons_outcome(SATISFIED)
 {
-	const auto parse_requirements = [&](const config& c, const std::string& id_key) {
-		if(c.has_attribute(id_key)) {
-			if(std::find(installed_addons.begin(), installed_addons.end(), c[id_key].str()) == installed_addons.end()) {
+	// Parse the list of addons required to join this game.
+	for(const config& addon : game.child_range("addon")) {
+		if(addon.has_attribute("id")) {
+			if(std::find(installed_addons.begin(), installed_addons.end(), addon["id"].str()) == installed_addons.end()) {
 				required_addon r;
-				r.addon_id = c[id_key].str();
+				r.addon_id = addon["id"].str();
 				r.outcome = NEED_DOWNLOAD;
 
 				// Use addon name if provided, else fall back on the addon id.
-				if(c.has_attribute("name")) {
-					r.message = vgettext("Missing addon: $name", {{"name", c["name"].str()}});
+				if(addon.has_attribute("name")) {
+					r.message = vgettext("Missing addon: $name", {{"name", addon["name"].str()}});
 				} else {
-					r.message = vgettext("Missing addon: $id", {{"id", c[id_key].str()}});
+					r.message = vgettext("Missing addon: $id", {{"id", addon["id"].str()}});
 				}
 
 				required_addons.push_back(std::move(r));
@@ -245,18 +246,6 @@ game_info::game_info(const config& game, const config& game_config, const std::v
 				}
 			}
 		}
-	};
-
-	for(const config& addon : game.child_range("addon")) {
-		parse_requirements(addon, "id");
-	}
-
-	/*
-	 * Modifications have a different format than addons. The id and addon_id are keys sent by the
-	 * server, so we have to parse them separately here and add them to the required_addons vector.
-	 */
-	for(const config& mod : game.child_range("modification")) {
-		parse_requirements(mod, "addon_id");
 	}
 
 	if(!game["mp_era"].empty()) {