Refactor RdfsImportEngine

[multimeric]

Built on top of #151 (so the diff will be simplified once that's merged).

Changes:

• Refactored RdfsImportEngine. I did this in the process of trying to understand the existing code and why it wasn't working.
  • Added type annotations everywhere
  • Added descriptions of instance variables
  • Refactored out the class and the slot generation into separate functions for clarity
• Fixed the issue with slots not correctly being assigned to classes (see Publish the compiled LinkML YAML? linkml-schemaorg#4). This was I think largely caused by the fact that we skipped any properties that weren't in the LinkML class metaschema, even when handling slots:
  

  schema-automator/schema_automator/importers/rdfs_import_engine.py

  Lines 191 to 192 in 2ae9ba2

  	if metaslot_name not in self.defclass_slots:
  	continue

This now creates a good enough LinkML schema for schema.org classes. If you run:

schemauto import-rdfs https://schema.org/version/latest/schemaorg-current-https.ttl

You get the attaced LinkML schema, which now has the appropriate slots.
schema.yml.txt