[BUG] DefaultCodeGen doesn't recurse into nested types when looking for imports

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?

Description

When specifying schemas that have nested types (for instance, Map<String, Map<String, MyType>>), the MyType custom type doesn't get added to the list of imports and so all the generated clients fail to resolve the MyType symbol in the related API. I noticed this with Python Client, but I believe it would effect others as well.

openapi-generator version

5.3.1

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Test additional properties with ref
  version: '1.0'
servers:
  - url: 'http://localhost:8000/'
paths:
  /ping:
    post:
      operationId: ping
      responses:
        default:
          description: default response
          content:
            application/json:
              schema:
                type: object
                additionalProperties:
                  type: object
                  additionalProperties:
                    type: object
                    additionalProperties:
                      "$ref": "#/components/schemas/Person"
components:
  schemas:
    Person:
      type: object
      properties:
        lastName:
          type: string
        firstName:
          type: string

Also here

Generation Details

A simple generation command, like openapi-generator generate -g python will make a Python client that fails during symbol resolution because Person won't be imported in default_api.py.

Steps to reproduce

Assuming pipenv and openapi-generator-cli are both installed and on the path...

openapi-generator-cli generate -g python -i https://gist.githubusercontent.com/ethan92429/07a09ac228dcd2f9ec0b957ddb44148b/raw/074beff13727280dd8119c3547ac36d28e58559b/openApi_generator_issue_11220.yaml &&
pipenv install -r requirements.txt &&
pipenv install -r test-requirements.txt &&
pytest --cov=openapi_client

Related issues/PRs

#10279

Suggest a fix

Working on this...

Start with the DefaultCodeGen import handling and reproduce the issue using the YAML declaration and openapi-generator-cli generate -g python command shown here. Run the generated client's pytest commands and verify that the nested Person reference is imported in default_api.py and symbol resolution succeeds.