cpconverter: Sling-Initial-Content directories created as nt:folder instead of sling:Folder

the cpconverter extracts Sling-Initial-Content from OSGi bundles and creates FileVault packages with the transformed content.

this works well, but there is one difference when the resulting content package is installed compared when uploading the OSGi bundle with the Sling-Initial-Content directly:

• the JCR Content Loader by defaults creates a sling:Folder node type for each directory found in the Sling-Initial-Content (see also docs)
• the cpconverter process creates no .content.xml file for the folders, but only for the actual JSON files found in the process. as a result, the folders are created as nt:folder when uploading the transformed package
• this difference becomes relevant, when a JSON file in Sling-Initial-Content defines a primary type of nt:unstructured - it is not allowed to created such a node directly below a nt:folder node - but it is allowed to do so below a sling:Folder node