[FEATURE]: Split OWL importer plugin into a package

[mnamici]

Description

Split the ontology importer plugin module into a python package to improve maintainability.

Motivation

Maintaining everything as a single 5K+ LOC module is not fun and makes maintaining the plugin harder.
Eddy supports package plugins since v1.2, we should take advantage of it.

Example Use Cases

A simple separation between the main sub-components should be enough already:

• a persistence-related module
• a module with the main worker stuff
• a module with the ui components