Fixed MULE-7260 better support for property placeholders in DataSense

* Fixed MULE-7260 adding better validation when there are multiple property placeholders
* Refactored SpringXmlConfigurationMuleArtifactFactoryTest to be in the same package as SpringXmlConfigurationMuleArtifactFactory
* Changed visibility of methods in SpringXmlConfigurationMuleArtifactFactory in order to make the class more testable