Describe the bug
When a schema in one file references another schema in another file using the $ref keyword, and the referenced schema uses the oneOf keyword in the OpenAPI specification, the model generated does not utilize the imports from the referenced schema.

For example. the schema SupportedCommands in commands.yaml has a $ref to the AddUserCommand schema in user.yaml. The AddUserCommand schema is using the oneOf keyword. The generated commands.py imports user as expected but doesn't use the import when referencing the user models

This bug only occurs when the models are generated using an input directory of OpenAPI schemas instead of a single openapi.yaml. It also only appears to be happening when I use the -collapse-root-models flag. When I generate without that flag, the imports are referenced correctly.

Generated models with unused imports

To Reproduce

1. Create input directory called schemas, containing two files, user.yaml and commands.yaml using the schemas below
2. Create output directory called models
3. Generate models using the datamodel-codegen command below in the root directory of the project

Example user.yaml schema:

openapi: 3.0.0
components:
  schemas:
    JsonRpcError:
      type: object
      properties:
        error:
          type: string
    User:
      type: object
      properties:
        userId:
          type: string
    AddUserCommand:
      oneOf:
        - $ref: "#/components/schemas/User"
        - $ref: "#/components/schemas/JsonRpcError"

Example commands.yaml schema:

openapi: 3.0.0
components:
  schemas:
    SupportedCommands:
      $ref: "user.yaml#/components/schemas/AddUserCommand"

Used commandline:

$ datamodel-codegen --input schemas --output models --collapse-root-models

Expected behavior
The user models referenced in commands.py should be correctly prefixed with the imported module name. For example, instead of User / JsonRpcError, it should be user.User / user.JsonRpcError, like shown below:

# generated by datamodel-codegen:
#   filename:  commands.yaml
#   timestamp: 2023-07-31T22:40:48+00:00

from __future__ import annotations

from pydantic import BaseModel

from . import user


class SupportedCommands(BaseModel):
    __root__: Union[user.User, user.JsonRpcError] 

Version:

• OS: Ubuntu 22.04.1 LTS
• Python version: Python 3.10.6
• datamodel-code-generator version: 0.21.2