Replace QFile::exists by QFileInfo::exists

QFile::exists only calls QFileInfo::exists, so this saves one
function call per invocation.

Change-Id: I41cb407345654eeead14455a4955b8b9b015aedc
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/plugininstallwizard.cpp

@@ -109,7 +109,7 @@ public:
     bool isComplete() const final
     {
         const FilePath path = m_data->sourcePath;
-        if (!QFile::exists(path.toString())) {
+        if (!QFileInfo::exists(path.toString())) {
             m_info->setText(Tr::tr("File does not exist."));
             return false;
         }