[BUG] [Java] [Retrofit2] [Play] Fix Retrofit2 templates for Play framework to use fully qualified package name

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

In the API templates for Retrofit2 with Play framework, api.mustache,
retrofit2.Response is imported but it is conflicted with the definition
of the response defined inline in the responses section and it does not
refer to the external definition by "ref" pointer. This causes the
compilation errors since the generated code are imported before
retrofit2.Response and they do not get along with the definition of
Retrofit2's Response class.

openapi-generator version

$ ./openapi-generator-cli.sh version                                                                         
4.2.3

OpenAPI declaration file content or url

https://support.sigfox.com/api/apidocs

Command line used for generation

$ cat config.json 
{
  "apiPackage":"com.example.openapi.client.api",
  "artifactId":"sample-openapi-client",
  "artifactVersion":"0.1.0",
  "dateLibrary": "java8",
  "groupId":"com.example",
  "hideGenerationTimestamp": true,
  "invokerPackage":"com.example.openapi.client.invoker",
  "java8": true,
  "library":"retrofit2",
  "modelPackage":"com.example.openapi.client.model",
  "playVersion": "play26",
  "serializationLibrary": "jackson",
  "usePlayWS": true
}
$ ./openapi-generator-cli.sh generate -i https://support.sigfox.com/api/apidocs -g java -c config.json -o tmp

Steps to reproduce

$ cat > config.json  <<EOF
{
  "apiPackage":"com.example.sigfox.client.api",
  "artifactId":"sigfox-api-client",
  "artifactVersion":"0.1.0",
  "dateLibrary": "java8",
  "groupId":"com.example",
  "hideGenerationTimestamp": true,
  "invokerPackage":"com.example.sigfox.client.invoker",
  "java8": true,
  "library":"retrofit2",
  "modelPackage":"com.example.sigfox.client.model",
  "playVersion": "play26",
  "serializationLibrary": "jackson",
  "usePlayWS": true
}
EOF
$ ./openapi-generator-cli.sh generate -i https://support.sigfox.com/api/apidocs -g java -c config.json -o tmp
$ cd tmp
$ chmod +x ./gradlew
$ ./gradlew build

The compilation in the last step fails with compile errors.

Related issues/PRs

None

Suggest a fix

I made a patch and I'm wiling to make a PR with it if this issue got accepted.

https://github.com/tfukushima/openapi-generator/commit/42e3c4144cca8d9a980c5f29fd181f5c898e22cc

Inspect api.mustache in the Java Retrofit2 templates for the Play framework, starting by reproducing the issue with the provided config.json and generation command. Run ./gradlew build in the generated project; done means the generated code compiles without the retrofit2.Response naming conflict and uses the fully qualified package name.