[Java] --model-name-prefix incompatible with oneOf

Description

--model-name-prefix in combination with oneOf produces Java code that does not compile.

Swagger-codegen version

3.0.21

Swagger declaration file content or url

openapi: 3.0.1                                                                                                                                                                 
info:                                                                                                                                                                          
  version: "1.0.0"                                                                                                                                                             
  title: Swagger Petstore                                                                                                                                                      
                                                                                                                                                                               
paths:                                                                                                                                                                         
  /pet:                                                                                                                                                                        
    post:                                                                                                                                                                      
      responses:                                                                                                                                                               
        '202'                                                                                                                                                                  
      requestBody:                                                                                                                                                             
        $ref: '#/components/requestBodies/Pet'                                                                                                                                 
                                                                                                                                                                               
components:                                                                                                                                                                    
  requestBodies:                                                                                                                                                               
    Pet:                                                                                                                                                                       
      content:                                                                                                                                                                 
        application/json:                                                                                                                                                      
          schema:                                                                                                                                                              
            $ref: '#/components/schemas/Pet'                                                                                                                                   
  schemas:                                                                                                                                                                     
    Pet:                                                                                                                                                                       
      type: object                                                                                                                                                             
      properties:                                                                                                                                                              
        baz:                                                                                                                                                                   
          oneOf:                                                                                                                                                               
            - $ref: "#/components/schemas/A"                                                                                                                                   
            - $ref: "#/components/schemas/B"                                                                                                                                   
          discriminator: discrim                                                                                                                                               
    A:                                                                                                                                                                         
      type: object                                                                                                                                                             
      properties:                                                                                                                                                              
        discrim:                                                                                                                                                               
          type: string                                                                                                                                                         
    B:                                                                                                                                                                         
      type: object                                                                                                                                                             
      properties:                                                                                                                                                              
        discrim:
          type: string

Command line used for generation

java -jar swagger-codegen-cli.jar generate -i petstore.yaml -l java --model-name-prefix=Foo

Steps to reproduce

Inspect the generated FooPet source model. This won't compile; it tries to import a non-existent model FoObject.

head -n24 src/main/java/io/swagger/client/model/FooPet.java | tail -n12
package io.swagger.client.model;

import java.util.Objects;
import java.util.Arrays;
import com.google.gson.TypeAdapter;
import com.google.gson.annotations.JsonAdapter;
import com.google.gson.annotations.SerializedName;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;
import io.swagger.client.model.FooObject;
import io.swagger.v3.oas.annotations.media.Schema;
import java.io.IOException;

Suggest a fix/enhancement

Fix imports: should not import FooObject. It could either import nothing instead, or perhaps instead import FooOneOfPetFooBaz (it's in the same package, but the rest of the codegen seems to import these anyways).

Reproduce the issue with the provided OpenAPI YAML and Java generation command, then inspect the generated src/main/java/io/swagger/client/model/FooPet.java import section. Trace the Java model/import generation path responsible for FooObject; done means the generated code compiles without importing a nonexistent model.