Reorganize package structure

[cmungall]

We have a flat list of things under

https://github.com/linkml/schemasheets/tree/main/schemasheets

• schema_exporter: linkml2sheets
• schemamaker: sheets2linkml
• schemasheet_datamodel
• sheets_to_project: primarily CLI
• schemaview_vs_example: ???

this is a little ad-hoc. For other projects we subdivide into packages, e.g.

• import
• export
• datamodel
• cli

This may be overkill here, but we should at least have consistent naming conventions, e.g. if schema_exporter goes from the metamodel to sheets, then the opposite should be called schema_importer

[sierra-moxon]

@turbomam - still a priority to fix? :)