Migrations for legacy and now illegal default link label _return

[sphuber]

Fixes #3527

The second commit adds the migration for export archives, however, it does not include the respective tests. @CasperWA would be willing to add those?

[CasperWA]

For future reference, I refer to: https://github.com/aiidateam/aiida-core/wiki/Checklist-when-updating-the-export-version

And I can add the files, sure 👍

[CasperWA]

Looks fine to me.

I have updated the aiida-export-migration-tests to v0.8.0 and updated the dependency here as well.
All tests should have been added, along with updating test archives to v0.8.

[ltalirz]

Don't wait for me - fine to go ahead

[sphuber]

Would be good if @giovannipizzi could still have a look to see if the migrations are ok.

[giovannipizzi]

I gave - I'm not fully comfortable with changing all link labels being _return (Maybe I would have limited to calcfunction and work function CREATE/RETURN links) - on the other hand there shouldn't be other people using this name, and now it's illegal so maybe ok?

This on the other hand raises a different question - should we migrate link labels of people that used an invalid one before validation is in place? Or it's ok to keep as is, and they just cannot create new ones? (I guess these are typically created by plugins, so plugin developers will anyway have to upgrade their plugins if using invalid names - the main question is if anything 'crashes' or raises if an invalid link is in the DB

[sphuber]

there shouldn't be other people using this name, and now it's illegal so maybe ok?

This was my thinking exactly. I don't think there is actual exceptions that can occur when just loading the node or links, but it will certainly prevent from importing archives that contain them, since then the link validation will stop the link from being created.

This on the other hand raises a different question - should we migrate link labels of people that used an invalid one before validation is in place? Or it's ok to keep as is, and they just cannot create new ones?

I don't think we can start to add data migrations for things done by plugins. We could remove leading and trailing underscores from link labels as that is what is now illegal, but not sure that we should. So far we have never considered plugin controlled data in data migrations and not sure if we should start now.