[BUG] OAS Validation should report an error when $ref uses '#/definitions'

Description

I'm using the gradle 5.2.1 plugin for validating my API

When my API references a type with a prefix of '#/definitions', I'd expect for the validator to report errors, but that's not the case

        - in: query
          name: pageable
          style: form
          explode: true
          schema:
            $ref: "#/definitions/Pageable"

Here is the output

./gradlew build

> Task :openApiValidate
Validating spec /home/adrian/dev/pai-api-v1/src/main/open-api/pai-api-v1.yaml
Spec is valid.

I get the right report (an error) when using the online tools (https://editor.swagger.io/ or https://apitools.dev/swagger-parser/online/):

Semantic error at paths./factures.get.parameters.1.schema.$ref
$refs must reference a valid location in the document

openapi-generator version

5.2.1

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Bla
  description: Bla
  version: "@VERSION@"
servers:
- url: '/api/v1'
paths:
  /factures:
    get:
      summary: liste des factures
      operationId: listerFactures
      parameters:
        - name: codesApprenants
          in: query
          description: les codes apprenants
          schema:
            type: array
            items:
              type: string
        - in: query
          name: pageable
          style: form
          explode: true
          schema:
            $ref: "#/definitions/Pageable"
      responses:
        200:
          description: liste des factures
          content:
            'application/json':
              schema:
                $ref: '#/components/schemas/Factures'
        404:
          description: Structure introuvable

components:

 schemas:

  Factures:
    properties:
      items:
        type: array
        items:
          $ref: '#/components/schemas/Facture'

  Facture:
    type: object
    properties:
      dateGeneration:
        type: string
        format: date
        description: La date de génération de la ventilation de droits.

Steps to reproduce

• unzip this file
  oas-validation-issue-def.zip
• run ./gradlew clean build

You should see the following report (no issue)

> Task :openApiValidate
Validating spec /home/adrian/dev/oas-validation-issue-def/sample.yaml
Spec is valid.

BUILD SUCCESSFUL

We would expect an error during validation

Start with the sample.yaml reproduction from the attached archive and run ./gradlew clean build, focusing on the openApiValidate task. Trace the validator entry point used by the Gradle 5.2.1 plugin and compare handling of the #/definitions/Pageable reference with the valid #/components/schemas references. Done means the build reports the invalid reference as an error.