[BUG] Using $ref under components/schema results in spec validation failure

A spec like this:

openapi: 3.0.3
info:
  title: api spec
  version: 1.0.0
servers:
  - url: /v3
components:
  schemas:
    $ref: './definitions/_index.yaml'
...

Results in validation error(s):

xception in thread "main" org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 2, Warning count: 0
Errors: 
	-attribute components.schemas.$ref is not of type `object`
	-attribute components.schemas.Schema name $ref doesn't adhere to regular expression ^[a-zA-Z0-9\.\-_]+$

	at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:544)
	at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:571)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:433)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

This is using docker.io/openapitools/openapi-generator-cli:latest (image id: e718a88b4de4). The spec with the schemas changed as below passes the validation:

openapi: 3.0.3
info:
  title: api spec
  version: 1.0.0
servers:
  - url: /v3
components:
  schemas:
    UnauthorizedError:
      $ref: "./definitions/unauthorized_error.yaml"
    NotFoundError:
      $ref: "./definitions/not_found_error.yaml"
...

According to 3.0 docs a $ref element should be valid immediately under schema.

Reproduce the validation failure with the OpenAPI 3.0.3 example, then trace the validation path from CodegenConfigurator.java through Generate.java and OpenAPIGenerator.java. Done means a components.schemas.$ref pointing to an external definitions file validates without requiring individually named schema entries.