Spike: how to import relations from LB3 models

[bajtos]

Find out what's needed to improve lb4 import-lb3-models to improve relations defined in LB3 models.

You can use LB3 built-in User model and its' has-many relation acsessTokens as an example.

Some of the questions to consider:

• How are we going to resolve the target model? E.g. when importing User model, how are we going to determine where to import AccessToken model from? What is the application does not have any AccessToken model defined and it is not selected for importing either? (See also Spike: how to import LB3 models extending a custom base model #3813 where we will be looking into this issue too.)

Acceptance criteria

• A PoC pull request demonstrating the proposed solution
• A spike document explaining which approaches have been tried, what worked and what didn't, etc.
• A list of follow-up tasks to implement the real version