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?
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Description

Using allOf with a $ref and an object, along with withSeparateModelsAndApi generates broken imports

openapi-generator version

5.1.0

OpenAPI declaration file content or url

openapi: 3.0.2
info:
  title: hello
  version: 1.0.0
components:
  schemas:
    SomeSchema:
      type: object
      properties:
        some_prop:
          type: integer
paths:
  /foobar:
    get:
      responses:
        200:
          description: ok
          content:
            application/json:
              schema:
                allOf:
                  - $ref: '#/components/schemas/SomeSchema'
                  - type: object
                    properties:
                      foobar: 
                        type: string

Generation Details

openapi-generator-cli generate -i http://localhost:8000/swagger.yml -g typescript-axios -o foo -p "withSeparateModelsAndApi=true,modelPackage=models,apiPackage=api"

Steps to reproduce

1. Run the above command with the provided yaml file
2. Look at the generated default-api.ts

On line 24, it creates an invalid import statement: import { SomeSchema & object } from '../models';