[PP-2] Update MigrationHelper and add hook_field_migration_field_widget_info() to add widget mappings.

Attached patch against #2

Sorry folks. There is already too many similar issues and patches out there.
Please never open issues without researching what's there already.
There's no issue summary either.

Adding this issue contributes to chaos, unnecessary work for others, adds bad karma, and negative contribution points.
Please do the clean up work.

Done.

Thanks Wim for chiming in and clarifying... partially.

> There is already too many similar issues and patches out there.
> Please never open issues without researching what's there already.

See parent issue.

Please note that the patch posted in #3 overlaps with the patches posted here: #3213595: MigrationHelper::alterPlugins assumes settings isn't already an array

But #3 in this issue additionally contains:

+/**
+ * Implements hook_field_migration_field_widget_info().
+ */
+function inline_entity_form_field_migration_field_widget_info(Row $row) {
+  if ($row->getDestinationProperty('field_type') === 'entity_reference') {
+     $addition = [
+       'inline_entity_form_single' => 'inline_entity_form_simple',
+       'inline_entity_form' => 'inline_entity_form_complex'
+     ];
+    return [
+      'entityreference' => $addition,
+      'commerce_line_item_reference' => $addition,
+      'commerce_product_reference' => $addition
+    ];
+  }
+  return [];
+}

which seems to be the key point here.

Did anyone review or test this already? Any suggestions in which of the issues to proceed?

!! Important information !!
(would have been nice if the patch authored had mentioned this .......)

All patches here in #2 and #3 are based on #3202462: [PP-1] Provide option for contrib modules to map their D6 / D7 field formatter and widget plugin IDs to the equivalent D9 plugin ID (CR DRAFT: https://www.drupal.org/node/3208917) which has not been committed yet. It's also postponed on #3204212: Convert remaining widget and formatter type migrations to MigrateField plugins !

This is why nothing will happen without also applying the core patches. So we should postpone this on these issues.

Furthermore, this will need work once the parent issue with the other changes is committed, for example this line is wrong:

           $settings[] = $migration['process']['settings'];
+          $settings = $migration['process']['settings'];

the above line has to be removed, but as this is also part of the parent / sibling issue, the future patch should ONLY contain the relevant changes here, which means the new hook implementation!

Thanks for the great cleanup work!