In .pro files the user can specify additional import paths using: QML2_IMPORT_PATH

In .qmlproject files the user can specify additional import paths using:
importPaths

The code model looks in the specified directories for QML plugins.
Using QML components from these plugins in the root document does work in Qt Quick Designer, because we instantiate the components directly from the source.

But those import paths are not forwarded to the qml(2)puppet and therefor they cannot be used indirectly. If for example a delegate uses such a component the whole delegate cannot be created and will not work.