Fix splitting artifact name property

[jhrozek]

Summary

If the artifact name had contained a slash, we would have failed to
parse it. Treat the owner as what's up to the first slash and the rest
as the artifact name.

Change Type

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

I used this artifact for testing: https://github.com/jakubtestorg/demo-repo-go/pkgs/container/demo-repo-go%2Fimage

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[JAORMX]

Should we also add named return parameters for documentation purposes?

[coveralls]

coverage: 52.867% (-0.006%) from 52.873%
when pulling 5c0bfad on jhrozek:fix_entity_lookup
into c8c9c1d on stacklok:main.