Handle entity_references related to Drupal 6 and 7 node translations with different IDs

Problem/Motivation

In Drupal 8, node translations are all stored in the same node as their default language node while in Drupal 6 & 7 they were stored in separate nodes. After a migration, entity reference field may now reference non-existent node translations, as show here:

Proposed resolution

Write a new deriver that will generate a migration for every entity bundle containing an entity reference field. These migrations will then update the entity reference field values with the new IDs found in the mapping tables using the migration_lookup process plugin.

Since those migrations need to be derived based on the migrated fields, we can't derived them at the same time as the other migrations since the fileds won't exists on the D8 site at that time. We need the migrations on which they depend to generate them after they have been succesfully executed.

Remaining tasks

Write the test and patch
Test it some more
Review it
Commit it!

User interface changes

N/A

API changes

New interface MigrationWithFollowUpInterface for migrations with follow-up migrations.

Data model changes

N/A