[BUG] `$ref` does not work under `paths/<uri>/<method>/parameters/<param>/schema`

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?

Description

According to OpenAPI Specification v3.0.3 $ref is allowed to define the schema of a Parameter object, however when doing so code generation generates warnings such as WARN o.o.codegen.utils.ModelUtils - #/components/schemas/... is not defined, while the $ref pointer is definitely correct, for example it can be resolved fine by a variety of other tools.

openapi-generator version

This behaviour is the same for both v5.0.0-beta2 and v4.3.1. I've also checked against the latest snapshot.

OpenAPI declaration file content or url

Full example:
https://gist.github.com/BenceSzalai/908243fbe9a3107a11bf9093c77e31bc

Relevant excerpt:

paths:
  /offer/:
    get:
      parameters:
        - name: country
          schema:
            $ref: '#/components/schemas/Offer/properties/country'
components:
  schemas:
    Offer:
      type: object
      properties:
        country:
          type: string

Steps to reproduce

1. Use the sample yaml provided above as the OpenAPI specification file
2. run the local equivalent of /bin/java -jar openapi-generator.jar generate -g php -i "sample.yaml" -o "generated" Actually the chosen generator doesn't matter, -g php is only an example.*
3. Observe [main] WARN o.o.codegen.utils.ModelUtils - #/components/schemas/Offer/properties/country is not defined emitted during generation.

*: In fact it exposes the issue more, since the $country parameter on DefaultApi::getOffer() in generated/lib/Api/DefaultApi.php gets type hinted with a non existent class: \OpenAPI\Client\Model\Country instead of string which should have been resolved from the $ref. But probably it has further consequences in other generators as well.

Start with the sample YAML and the local openapi-generator.jar generate -g php command to reproduce the warning from ModelUtils. Trace how the $ref under the parameter schema is resolved, then inspect the generated generated/lib/Api/DefaultApi.php; done means the reference resolves without a warning and the parameter is type hinted as string rather than a nonexistent Country model.