Don't permit requesting terms cross routes

I have a category with `term_taxonomy_id=1`:

```
GET http://wordpress-develop.dev/wp-json/wp/v2/terms/category/1
```

I shouldn't be able to request it from the `tag` endpoint:

```
http://wordpress-develop.dev/wp-json/wp/v2/terms/tag/1
```

Reported in https://wordpress.org/support/topic/incorrect-route-wordpress-44-beta-4?replies=5
