Update transkribus import file detection to support pageDirName

refs https://gitlab.com/arkindex/doc/-/issues/62

Client can generate an export with pageDirName not with our usual (empty) value. So the transkribus import code must upgrade its internal logic to match images & XML files.

A first method should be implemented to support both mode:

• when pagedirname is empty
• when pagedirname is set, and the XML files are in a sub folder

The import code would then loop on the results of that method and import each element/image combo.

The correct way to do that is using METS import (but that's another longer dev)