make qtbase source requirement explicit

getting rid of QT.<module>.sources references, as they don't go well
with real modularization.

Change-Id: Idfe0327b00be173e2c2a0b4fc4d2c50de7c2dbe8
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

examples/designer/arthurplugin/arthurplugin.pro

@@ -23,8 +23,8 @@ contains(QT_CONFIG, opengl) {
     QT += opengl
 }
 
-QT_BASE_EXAMPLES = $$QT.core.sources/../../examples
-SHARED_FOLDER = $$QT_BASE_EXAMPLES/widgets/painting/shared
+requires(!isEmpty(_QMAKE_SUPER_CACHE_))
+QT_BASE_EXAMPLES = ../../../../qtbase/examples
 include($$QT_BASE_EXAMPLES/widgets/painting/shared/shared.pri)
 
 EXAMPLE_AFFINE_DIR = $$QT_BASE_EXAMPLES/widgets/painting/affine