Create relationships based on scalar computed fields.

### Is your proposal related to a problem?

Users may return scalar values such as a user id from computed fields,
but those are not able to be used to define relationships.

### Describe the solution you'd like

Users should be able to use computed fields in relationships just like any other field.

### Describe alternatives you've considered

Users can create computed fields that return a set of records, effectively an array relationship.
Currently we cannot return a single item (object relationship). That feature request is being tracked in #6562

No files, tests, or entry points are identified in the issue. Start by reviewing the related issue #6562 and tracing the existing computed-field and relationship handling; done means scalar computed fields can define single-item relationships as well as collection relationships.

Written by the indexing model from the issue text.