Field collections deriver and base migration

Problem/Motivation

We need a method to upgrade/migrate from D7 field collections to D8 paragraphs.

Proposed resolution

Field Collections => Paragraphs deriver and base migration using Node as a pattern.

Remaining tasks

1. 📌 Alter derived migration definition early in life Needs work
2. #2890690: MigrateLookup plugin has inconsistent return values. →
3. #2954937: Object of class FieldCollectionItem could not be converted to int in FieldCollectionItem->fields() →
4. The latest dev version of Entity Reference Revisions → , the patch from #2944836: Migrate destination plugin attempts to update new entities, preventing creation of stub entities → , or a release that contains it.
5. Document example migrations, such as data only for a single Field Collection. We can generate a template from #30, #32 and the extra process (see later comment) for migrating the IDs as well.

User interface changes

API changes

Data model changes

Follow-up issues

• No ticket yet: Add support for ✨ Support asymmetric translation in experimental widget Needs review when it gets in.