[JAVA] Nested generics causes missing imports with openapi 3.0

Description

When a method is using nested generics in an method argument, like Map<String, List<UUID>>, the generated Api class is missing an import for this model class. In this case 'UUID'.

Swagger-codegen version

3.0.27

Swagger declaration file content or url

definition in controller

  @GetMapping(value = "/running", produces = MediaType.APPLICATION_JSON_VALUE)
  public ResponseEntity<Map<String, List<UUID>>> getAllRunningApps() {
    Map<String, List<UUID>> runningApps = diagnosticService.loadRunning();
    return ResponseEntity.ok().eTag( HashUtils.hash( runningApps ) ).body( runningApps );
  }

Json for code generating:

{
  "openapi": "3.0.1",
  "info": {
    "title": "Server",
    "contact": {
      "url": "http://google.com",
      "email": "info@google.com"
    },
    "version": "0.0.20-5"
  },
  "servers": [
    {
      "url": "http://127.0.0.1:9018",
      "description": "Generated server url"
    }
  ],
"paths": {
"/rest-ws/diagnostic/v1/app/running": {
      "get": {
        "tags": [
          "AppsController V1"
        ],
        "summary": "Retrieves all running applications in map with computer name asi key",
        "operationId": "getAllRunningApps",
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "application/json": {
                "schema": {
                  "type": "object",
                  "additionalProperties": {
                    "type": "array",
                    "items": {
                      "type": "string",
                      "format": "uuid"
                    }
                  }
                }
              }
            }
          },
          "413": {
            "description": "Payload Too Large",
            "content": {
              "*/*": {
                "schema": {
                  "type": "object"
                }
              }
            }
          },
          "422": {
            "description": "Unprocessable Entity",
            "content": {
              "*/*": {
                "schema": {
                  "type": "object"
                }
              }
            }
          },
          "501": {
            "description": "Not Implemented",
            "content": {
              "*/*": {
                "schema": {
                  "type": "object"
                }
              }
            }
          }
        }
      }
    }
  }
}

In generated class it is missing import java.util.UUID;

Steps to reproduce

Bug is also reproducible in Swagger Editor

• open https://editor.swagger.io/
• paste given json snapshot into it
• Generate Client and choose java
• Download and extract given project
• open class in AppsControllerV1Api.java in folder src\main\java\io\swagger\client\api
• you will find out, that import java.util.UUID is missing, so compiler is very unhappy

I use swagger-codegen-maven-plugin with very same results when I use resttemplate library.

Related issues/PRs

This issue was probably solved for OpenApi 2 in issue:
https://github.com/swagger-api/swagger-codegen/issues/5885

Suggest a fix/enhancement

Maybe there is missing recursive search in collections for import classes.

Start by reproducing the generated AppsControllerV1Api.java result in Swagger Editor or with the swagger-codegen-maven-plugin using the nested Map<String, List> response. Compare nested import handling with related issue 5885; done when java.util.UUID is emitted and the generated client compiles.