Migration destination lookup includes ignored rows, can break lookup with multiple migrations

Problem/Motivation

When using the migration_lookup process plugin and providing multiple migrations to search in, the plugin may exit early due to NULL destination ids from skipped rows being returned by the SqlMap plugin.

Example setup:

• A migration lookup using multiple migrations
  
  

  -
    plugin: migration_lookup
    migration:
      - node_by_x
      - node_by_y
      - node_by_z
    no_stub: true
    source: nid
  

• Source ids are valid for multiple of these migrations
• Each migration is only responsible for a subset of its source rows, so these migrations have some ignored rows in the map
  

  _valid:
    -
      plugin: static_map
      map:
        12: true
        123: true 
        1234: true
      default_value: ''
      source: uid
    -
      plugin: skip_on_empty
      method: row
  

This code in MigrationLookup::transform is perfectly fine:

      // Break out of the loop as soon as a destination ID is found.
      if ($destination_ids = $migration
        ->getIdMap()
        ->lookupDestinationId($source_id_values[$migration_id])) {
        break;
      }

However, in a setup where a migration may skip rows, the Sql::lookupDestinationIds method will treat skipped rows as if they were imported, but with NULL destination id(s), breaking any further lookups for migrations that may have actually imported the row.

Proposed resolution

Only return rows from the migrate map that were handled by the migration (= are not ignored) in Sql::lookupDestinationIds.

As a workaround, we've been chaining lookups for each migration individually, using a custom processor to check for empty destination ids.

Remaining tasks

Review, add tests.
Optionally decide whether other map row statuses should be excluded as well (e.g. failed rows).