Support forum taxonomy terms

This is a follow-up to the patch I created for Migrate d2d: https://www.drupal.org/project/migrate_d2d/issues/3002973 ✨ Support forum taxonomy terms Needs review I believe both patches are necessary.

Let me start by saying I am open to taking a different approach, but the patches I have created do work.

The fundamental issue is MigratePathEntityHandler makes the assumption that all taxonomy terms are the same. However, this is not the case.

The Forum module included in core bypasses what the Taxonomy module does, and creates a path of forum/tid for terms that are part of the "forums" vocabulary, rather than taxonomy/term/tid. In fact, forums terms *never* have a path of taxonomy/term/tid stored in the url_alias table.

The conflict with the Migrate module is how it handles creation of paths. The import function in MigrateDestinationTerm relies on taxonomy_term_save, as I would expect. The Migrate module also expects a value for $path to exist, otherwise a path will not be created.

However, having a $path value set causes the path_taxonomy_term_insert function to run, and a taxonomy/term/tid path is created. On the other side, the value of $term->path is not set when creating a forums term, and therefore the creation of a taxonomy/term/tid path is properly skipped.

So I'm not sure the best way to handle this... Migrate needs a $path to generate paths, but Forum taxonomy terms cannot have $path set, or else two entries are made to the url_alias table (one for taxonomy/term/tid and a second for forum/tid.

The best solution I could come up with at the moment is to perform a check in the complete function, and delete the taxonomy/term/tid path when a forum term is created.

I'd appreciate any feedback and review on the attached patch. Thanks.