Update target more granularly

If I remember correctly, there are the following challenges in play:

1. Feeds cannot always make the distinction between empty vs non-existent. This was stated in #1107522-147: Framework for expected behavior when importing empty/blank values + text field fix → .
   We should test whether or not this is still true for the current version of Feeds or if this was the case for the D7 version of Feeds only.
2. Mapping multiple times to the same target (a way to import multiple values for the same field).
   If the first mapped source is non-existent, will the target still be emptied when the second mapped source does exist?
3. Expectations of the current behavior.
   Right now it is ensured that all mapped targets will get overwritten. So I think we should at least keep supporting that workflow, in order to not break existing import workflows that rely on that behavior.

Other things to take in consideration: I'm about to open an issue of moving part of the implementation of clearTarget() to the target plugin. A patch has already been written.

So if we're going to support this workflow, we should at least make it configurable. For the D7 version of Feeds, there was the module Feeds empty → , that allowed to configure the behavior per target.
I'm not sure if we should still make this configurable per target or if it should be a one on-off switch.

Adding 📌 Move clearing the target from the processor to the target plugin Fixed as a related issue as it can have influence on how a solution for this issue is implemented.

@RoSk0
This does look like a duplicate of ✨ Feeds "update existing" - how to avoid wiping out all existing data? Active . Could you update that other one with your thoughts and then close this as a duplicate?