[BUG][Golang][Client] `composed-OneOf` become undefined

Description

composed-oneOf generates undefined type OneOfObjA.
here is generated code
this api should return one of ObjA or ObjB.
but Golang doesn't have such feature to describe TypeA | TypeB.

FYI https://developers.google.com/protocol-buffers/docs/reference/go-generated#oneof

openapi-generator version

4.0.0-SNAPSHOT 9e1cce2e7

OpenAPI declaration file content or url

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/resources/3_0/composed-oneof.yaml

Command line used for generation

generate -t modules/openapi-generator/src/main/resources/go -i modules/openapi-generator/src/test/resources/3_0/composed-oneof.yaml -g go -o $GOPATH/src/github.com/zigen/go-sample-api -DpackageName=go_sample_api
https://github.com/zigen/openapi-generator-go-client-oas3-repro/blob/master/go-oneof.sh

Steps to reproduce

generate spec modules/openapi-generator/src/test/resources/3_0/composed-oneof.yam to golang and import it.

this repo includes steps to repro, generated conde and example client.
https://github.com/zigen/openapi-generator-go-client-oas3-repro

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/1360
https://github.com/OpenAPITools/openapi-generator/issues/500

Suggest a fix

Start with modules/openapi-generator/src/test/resources/3_0/composed-oneof.yaml and the Go generator templates under modules/openapi-generator/src/main/resources/go. Run the reported generate command or go-oneof.sh from the linked reproduction repository and inspect the generated api_default.go around the undefined OneOfObjA type. Done means the composed-oneOf specification generates a Go client that can be imported without the undefined type error.