refactor: put resource downloading classes in common namespace

Puts them all inside the 'ResourceDownload' namespace, so that it's a
bit clearer from the outside that those belong to the same 'module'.

Signed-off-by: flow <flowlnlnln@gmail.com>

launcher/ui/pages/instance/ModFolderPage.cpp

@@ -158,7 +158,7 @@ void ModFolderPage::installMods()
         return;
     }
 
-    ModDownloadDialog mdownload(this, m_model, m_instance);
+    ResourceDownload::ModDownloadDialog mdownload(this, m_model, m_instance);
     if (mdownload.exec()) {
         ConcurrentTask* tasks = new ConcurrentTask(this);
         connect(tasks, &Task::failed, [this, tasks](QString reason) {