Rewrite plugin doesn't replace "parent" values

Same issue seems resolved and RTBC for Drupal 7: https://www.drupal.org/project/feeds_tamper/issues/2632056 ✨ Rewrite value using another field value only if empty. Needs work

I plan to work on this in the next few weeks.

I've provided a solution for this issue! Even though the tests are not passing yet, this is now ready for review and testing.

It works as follows:

1. Tamper plugins that work with a tamperable item (like the Rewrite plugin), provide a list of source fields they use on the item.
2. Feeds Tamper checks if any of these source fields are provided by FeedsSource plugins. This is the case for all "Feed entity" sources, that are listed for the Rewrite plugin as "[parent:*]" tokens.
3. Feeds Tamper loads the data for FeedsSource source fields.

Be sure to test it in combination with 📌 Make Tamper plugins tell which properties they use of a tamperable item Active . That issue provides changes for the Tamper module, so that plugins can provide a list of source fields that they use. Changes don't apply on the latest Tamper release, so you should apply them on the dev release. The Tamper dev release also contains other improvements for the Rewrite plugin.