Update imports generated in java layer to use one helper for all locations

Created by: spacether

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
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Description

This is needed to ensure the referenced schemas in not are added to the model/endpoint imports import missing for ref in not

Feature: jsonschema.$ref/not

openapi-generator version

6.0.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: openapi 3.0.3 sample spec
  description: sample spec for testing openapi functionality, built from json schema
    tests for draft6
  version: 0.0.1
paths: {}
components:
  schemas:
    PropertyNamedRefThatIsNotAReference:
      properties:
        $ref:
          type: string
    RefInNot:
      not:
        $ref: '#/components/schemas/PropertyNamedRefThatIsNotAReference'

In RefInNot there is no import for PropertyNamedRefThatIsNotAReference

Generation Details

python-experimental

Steps to reproduce

generate w/ the above generator

Related issues/PRs

Found when working on: https://github.com/OpenAPITools/openapi-generator/pull/12619

Suggest a fix

Add the import in defaultcodegen.java