Entity reference fields in paragraphs type cannot be selected as target

Problem/Motivation

I have a Paragraphs Type that has 2 number fields and 2 entity reference fields. In the Feeds Mapper I can select the number fields as targets, but not the entity reference fields.

Steps to reproduce

- create content type Diagram with some fields
- create paragraph type diagram-in-book
- add number field Page
- add entity reference field Diagram
- create content type Book
- add paragraph field table-of-contents
- create feed type diagram-in-book
- add processor Node
- add content type Book
- add mapping
- select table-of-contents field as target
- configure to use paragraph type diagram-in-book
- save
- select target field Page in table-of-contents

It is not possible to select field Diagram in table-of-contents

Proposed resolution

It should be possible to select entity_reference type targets, because that's also possible if I just import the paragraph using the available paragraphs mapper in the Feeds module itself.

Remaining tasks

User interface changes

API changes

Data model changes