QmlJS: Fix running qmldump on plugins that require a specific uri.

The builtin QML plugins require to be imported with the full uri, i.e.
import Qt.labs.particles 1.0
so setting the import path to imports/Qt/labs and doing
import particles 1.0
is not supposed to work. (see QTBUG-11139)

This change adjusts qmldump to take an import path *and* the import uri,
so it will be able to dump the type information for these plugins.

Reviewed-by: Erik Verbruggen