Draft: Creating ModelVersion also creates Package

What does this MR do and why?

Creating ModelVersion also creates Package

Beforehand the package was only created when a model version was created through the ml_model package endpoint, which would cause model versions created in other places to be inconsistent. While a nil value for package should be allowed (deleted in cleanup job for example), at creation time it should always be available.

How to set up and validate locally

1. Create a model version with the service

   project = Project.find_by(id: 1)
   user = User.find_by(id: 1)
   params = {user: user, model_name: 'blah', version: '1.2.3'}
   
   mv = Ml::FindOrCreateModelVersionService.new(project, params).execute

2. Verify that a package was created:

   mv.package

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #428905 (closed)