Fix 'The Drupal\migrate\Plugin\migrate\process\Migration is deprecated in Drupal 8.4.0 and will be removed before Drupal 9.0.0. Instead, use Drupal\migrate\Plugin\migrate\process\MigrationLookup'

1. +++ b/core/modules/block/src/Plugin/migrate/process/BlockPluginId.php
   @@ -19,13 +20,27 @@
   +   * @deprecated in drupal:8.8.x and is removed from drupal:9.0.0. Use
   +   *   migrateLookup instead.
   

   I think we should make it clear that we suggest folks use the migrate.lookup service now.

2. +++ b/core/modules/filter/src/Plugin/migrate/process/d6/FilterFormatPermission.php
   @@ -60,10 +94,23 @@ public static function create(ContainerInterface $container, array $configuratio
   +        // This BC layer is included because if the plugin constructor was called
   +        // with a migration process plugin, it may have been preconfigured with a
   

   Nit: 80 characters.

3. +++ b/core/modules/migrate/src/Plugin/migrate/process/MenuLinkParent.php
   @@ -75,16 +126,27 @@ public function transform($value, MigrateExecutableInterface $migrate_executable
   +    // This BC layer is included because if the plugin constructor was called
   +    // with a migration process plugin, it may have been preconfigured with a
   +    // different migration to look up against.
   

   The wording of this comment doesn't make sense to me. We've got too many plugins called 'migration'. Are we meaning the migration lookup service, the plugin, migration base plug or the legacy migration process plugin? This isn't clear.

I applied the patch and see things like:

            if ($this->migrationPlugin) {
              $block_id = $this->migrationPlugin
                ->transform($delta, $migrate_executable, $row, $destination_property);
            }
            else {
              $lookup_result = $this->migrateLookup->lookup(['d6_custom_block', 'd7_custom_block'], [$delta]);
              if ($lookup_result) {
                $block_id = $lookup_result[0]['id'];
              }
            }

This means we have hard coded migration names in the lookups. This isn't a good thing as it leads to BC concerns for folks using migrate_upgrade. Am I missing something though and that isn't a concern? Traditionally, we've steered away from doing this.

#3091841: Remove hardcoded plugin IDs from migration process plugins is opened to address #23. As discussed in Slack, that is an existing issue that is outside of scope of this. Let's pass this on to RTBC.

Committed and pushed efc4dd9796 to 9.0.x and e8b6250313 to 8.9.x. Thanks!

+++ b/core/tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php
@@ -122,7 +122,6 @@ public static function getSkippedDeprecations() {
-      'The Drupal\migrate\Plugin\migrate\process\Migration is deprecated in Drupal 8.4.0 and will be removed before Drupal 9.0.0. Instead, use Drupal\migrate\Plugin\migrate\process\MigrationLookup',

<3

Discussed with @catch and we agreed to backport.