refactor: separate polymorphic functions

[bf4]

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

Extracted from #1424
since it hints at a general usefulness of having a dedication function
module for handling polymorphic lookups.

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[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