[Java-Playframework][Core] Referenced models not in model list

Description

We have bumped into a situation where a model ends up as an import to another model, but it does not show up in the list of CodegenModels. This happens because one of our models is an allOf which references a model called ModelError, but ModelError is not directly used anywhere in our api definition.

In the example below, ModelError ends as an import for InvalidArgument, but ModelError is not in the list of models.

I am tagging this as Java-Playframework because that's where I've seen the error surface, but it looks to me like a problem in DefaultCodegen

Swagger-codegen version

2.3.0-SNAPSHOT

Swagger declaration file content or url

File 1:

[...]
"/path": {
      "get": {
        "responses": {
          "200": {
            "description": "OK",
            "schema": {
              "$ref": "#/definitions/ConsumptionsData"
            },
          "400": {
            "description": "Problem with the client request, in this case both OUT_OF_RANGE and INVALID_ARGUMENT are possible",
            "schema": {
              "$ref": "../common/errors.json#/definitions/InvalidArgument"
            },
            "examples": {
              "application/json": {
                "code": "OUT_OF_RANGE",
                "message": "Client specified an invalid range"
              }
            }
          }
        }
      }
    },
[...]

File 2:

{
  "definitions": {
    "ModelError": {
      "type": "object",
      "required": [
        "message"
      ],
      "properties": {
        "message": {
          "type": "string",
          "description": "A human readable description of what the event represent"
        }
      }
    },
    "InvalidArgument": {
      "allOf": [
        {
          "type": "object",
          "required": [
            "code"
          ],
          "properties": {
            "code": {
              "type": "string",
              "enum": [
                "INVALID_ARGUMENT"
              ],
              "default": "INVALID_ARGUMENT",
              "description": "Client specified an invalid argument, request body or query param."
            }
          }
        },
        {
          "$ref": "#/definitions/ModelError"
        }
      ]
    }
}

Start in DefaultCodegen and trace how CodegenModels is populated for allOf references, using the two Swagger definition files and the Java-Playframework path described in the issue. Done when the referenced ModelError appears in the model list as well as being imported for InvalidArgument.