Identify any gaps with compatibility with popular modules compared to core Entity Reference

Problem/Motivation

If Primary Entity Reference is not as well supported as Entity Reference, we should identify what the gaps are and prioritize appropriately.

Steps to reproduce

Proposed resolution

Research led to creation of these issues (in proposed priority order):

1. Views: #3558094: Views support
2. Token: #3558159: Better token support
3. Rules: #3559155: Rules (Typed Data?) support for the primary item
4. Feeds: #3559173: Feeds support
5. (modules providing entity reference field formatters/widgets): #3559176: Support for Field Formatters and Field Widgets defined for core Entity Reference fields
6. Corresponding Entity References: (CER issue) #3558164: Add support for Primary Entity Reference fields (and others that extend Entity Reference?)

Other modules evaluated, but for which no action seems necessary:

• Search API: no action required; Search API supports any field type that extends EntityReferenceItem
• ECA: leverages Token so its issue (above) should mostly suffice
• Entity Browser: leverages Views so its issue (above) will suffice

There are presumably a bunch of modules (like CER, above) that hardcode special support for core entity reference fields and which would need to take action to support Primary Entity Reference. Presumably those would be handled on an as-needed basis in their respective issue queues.

Remaining tasks

User interface changes

API changes

Data model changes