Improve robustness of QML plugin loading

Use a variable that expands to the latest plugin version instead of
hard-coding the string.

Change-Id: I8f7864e4ead85487943ed2c4dc2ae2584005c1c9
Reviewed-by: Paolo Angelelli <paolo.angelelli@theqtcompany.com>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

src/imports/location/location.cpp

@@ -83,7 +83,7 @@ class QtLocationDeclarativeModule: public QQmlExtensionPlugin
 {
     Q_OBJECT
 
-    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QQmlExtensionInterface/1.0"
+    Q_PLUGIN_METADATA(IID QQmlExtensionInterface_iid
                       FILE "plugin.json")
 
 public:

src/imports/positioning/positioning.cpp

@@ -506,7 +506,7 @@ class QtPositioningDeclarativeModule: public QQmlExtensionPlugin
 {
     Q_OBJECT
 
-    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QQmlExtensionInterface/1.0"
+    Q_PLUGIN_METADATA(IID QQmlExtensionInterface_iid
                       FILE "plugin.json")
 
 public:

tests/plugins/declarativetestplugin/locationtest.cpp

@@ -48,7 +48,7 @@ static QObject *helper_factory(QQmlEngine *engine, QJSEngine *scriptEngine)
 class QLocationDeclarativeTestModule: public QQmlExtensionPlugin
 {
     Q_OBJECT
-    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QQmlExtensionInterface/1.0")
+    Q_PLUGIN_METADATA(IID QQmlExtensionInterface_iid)
 public:
     virtual void registerTypes(const char* uri)
     {