There are a number of abstract methods here, but there appears to be no guidance / examples on how to implement these methods in your code. I have some code which used to work but does not now as extra abstrct methods have been added.eg GetFolders()
The latest XML Schema, allows to define a Directory node, so you can deploy to SP an entire folder (applying filters and custom metadata). The GetFolders uses that Directory node, and if you set it as Recursive, GetFolders get the sub folders (I've only seen the FileSystem implementation). If you don't need it, just return an Empty List<string> and it should work.