[BUG] [python-flask] Polymorphism causes imports to non-existent models

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When attempting to use polymorphism as described here in the specification controllers will contain import statements to models which have not been generated

openapi-generator version

Tested with 4.1.3, as well as snapshots of 4.2.1 and 5.0.0. Issue exists in all tested versions.

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Python polymorphism bug
  version: 1.0.0
paths:
  "/foo":
    post:
      responses:
        "200":
          description: "Success"
          content:
            application/json:
              schema:
                anyOf:
                  - $ref: "#/components/schemas/Bar"
                  - $ref: "#/components/schemas/Baz"
components:
  schemas:
    Foo:
      type: object
      properties:
        prop1:
          type: string
    Bar:
      allOf:
        - $ref: "#/components/schemas/Foo"
        - type: object
          properties:
            prop2:
              type: string
    Baz:
      allOf:
        - $ref: "#/components/schemas/Foo"
        - type: object
          properties:
            prop3:
              type: string

Command line used for generation

java -jar openapi-generator-cli-4.1.3.jar generate -g python-flask -i openapi.yml

Steps to reproduce

1. Copy above yaml to openapi.yml
2. Run command from above

openapi_server/controllers/default_controller.py now contains the line
from openapi_server.models.any_of_bar_baz import AnyOfBarBaz. AnyOfBarBaz is not a model which has been generated.

Start with the reproduced command and inspect openapi_server/controllers/default_controller.py, especially the generated import of AnyOfBarBaz. Trace how the anyOf schema is mapped to model imports, then verify the generated output no longer imports a model that was not generated using the supplied OpenAPI document.