[BUG] [kotlin] $refs to separate files are broken

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

$refs are not followed when referencing local files

openapi-generator version

5.3.0

OpenAPI declaration file content or url

openapi: "3.0.2"
info:
  title: API Title
  version: "1.0"
servers:
  - url: https://api.server.test/v1
paths:
  /test:
    post:
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Request'
      responses:
        '200':
          description: OK

components:
  schemas:
    Request:
      $ref: './schemas/request.yaml'

Request:
  type: object
  properties:
    someProp:
      type: string

Generation Details

Maven plugin with kotlin generator with following config:

<configuration>
      <inputSpec>openapi/openapi.yaml</inputSpec>
      <generatorName>kotlin</generatorName>
      <generateApis>true</generateApis>
      <generateModels>true</generateModels>
</configuration>

Steps to reproduce

Run the openapi-generator-plugin, see that the generated API file is using kotlin.Any as a method parameter instead of a Request object. Request.kt is not even generated.

Suggest a fix

Follow $refs to other files from main file.

Start with the Maven plugin configuration and Kotlin generator using the provided main OpenAPI file and schemas/request.yaml. Reproduce the generation and trace how the local $ref is resolved. Done means Request.kt is generated and the API method uses Request instead of kotlin.Any.