[BUG] Missing model type declarations in generated Go code

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issuue still exists?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

I use the openapitools/openapi-generator-cli docker image to generate a Go package. With the latest release I get errors complaining about undefined types when I try to use the autogenerated package.

This was not an issue with v4.3.1.

I used the Petstore example in the README.md file and see the same behavior.

Basically in the Petstore example there is a struct type called Pet. It is used in various places in the generated code such as here in api_pet.go:

func (r ApiFindPetsByStatusRequest) Execute() ([]Pet, *_nethttp.Response, error) {

But its not defined anywhere.

In v4.3.1 it is defined in a file called model_pet.go, but there are no model_ files generated with v5.0.0.

openapi-generator version

v5.0.0

OpenAPI declaration file content or url

https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml

Generation Details

Steps to reproduce

Using example in README file:

docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:latest generate -i https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml -g go -o /local/out/go

No Pet type defined:

> grep -rI "type Pet" ./out/go/
./out/go/api_pet.go:type PetApiService service
> 

But used in multiple places:

> grep -rI "Pet)" ./out/go/
./out/go/api_pet.go:func (r ApiAddPetRequest) Body(body Pet) ApiAddPetRequest {
./out/go/api_pet.go:func (r ApiUpdatePetRequest) Body(body Pet) ApiUpdatePetRequest {
...

With 4.3.1:

> sudo rm -rf out
> docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:v4.3.1 generate -i https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml -g go -o /local/out/go

Pet defined in model_pet.go:

> grep -rI "type Pet " ./out/go/
./out/go/model_pet.go:type Pet struct {
> 

Related issues/PRs

Suggest a fix

Start with the README generation command and the Petstore spec at modules/openapi-generator/src/test/resources/2_0/petstore.yaml. Compare generated api_pet.go with the v4.3.1 output, especially the missing model_pet.go and references to Pet. Done means the Go generator again emits model declarations usable by the generated API code and the reproduction no longer reports undefined types.