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

Created by: tfukushima

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
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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