Write an XML Schema importer

[cmungall]

This would be the reverse operation to: linkml/linkml#117

(currently importers live in schema-automator and generators in the main linkml, we may want to consider migrating, as these mature)

@ialarmedalien if you were inclined to take this on, I would use SqlImportEngine as a template (or JsonSchemaImportEngine, but that one is a bit messy and over-complex and may need to be rewritten - SqlImportEngine is simple and clean)

[ialarmedalien]

There are a couple of python libraries I have looked at / tinkered with that can read XSD files and generate a corresponding set of python dataclasses or templated source code. Converting the output of one of those modules to be compatible with linkml python objects would save a lot of work.