refactor: separate polymorphic functions

[bf4]

• refactor: polymorphic lookups to utility module
• refactor: lookup polymorphic types only once

Extracted from https://github.com/cerebris/jsonapi-resources/pull/1424
since it hints at a general usefulness of having a dedication function
module for handling polymorphic lookups.

All Submissions:

• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
• My submission passes all tests. (Please run the full test suite locally to cut down on noise from travis failures.)
• I've used Github auto-closing keywords in the commit message or the description.
• I've added/updated tests for this change.

New Feature Submissions:

• I've submitted an issue that describes this feature, and received the go ahead from the maintainers.
• My submission includes new tests.
• My submission maintains compliance with JSON:API.

Bug fixes and Changes to Core Features:

• I've included an explanation of what the changes do and why I'd like you to include them.
• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions

[bf4]

👀 named Utils because my intention is for this to be a collection of functions which may not have an obvious theme or place on their own.
Called extend self instead of module_function to declare that this module is intended to hold functions, not to be included anywhere.

[bf4]

now refactoring the way we build the polymorphic lookup doesn't add any interface to the JSONAPI::Relationship nor JSONAPI::Resource