[Bug] Golang generator should use tags as package names

[wtrocki]

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 issue still exists?
• Have you searched 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

Golang generator should generate multiple packages similar to java operator.
Package should be based on the tags:

Currently generating open API for golang will end up with errors related to duplicates.
For example generating this API will create +30 errors in methods and structs duplication:
https://github.com/Apicurio/apicurio-registry/blob/2.0.0.Final/app/src/main/resources-unfiltered/META-INF/resources/api-specifications/registry/v2/openapi.json#L2952-L2983

openapi-generator version

master and 5.1.1

OpenAPI declaration file content or url

https://github.com/Apicurio/apicurio-registry/blob/2.0.0.Final/app/src/main/resources-unfiltered/META-INF/resources/api-specifications/registry/v2/openapi.json

Generation Details

openapi-generator-cli generate -i openapi.json -g go --package-name srsdata -p="generateInterfaces=true" -o ./api

Steps to reproduce

1. Generate API from spec
2. Generated code will not compile because of many duplicated elements

Suggest a fix

After checking implementation I think there is reasonable way to fix it:

• Work on merging model placement [REQ] [go-gin-server] place models in different package to prevent circular package references  #2752
• When tags are present (non default tags) their names should be used as package names to prevent duplicates.

I'm happy contribute this change considering it is something that is valid for golang and openapi gen community.

[wtrocki]

See also: #1931

[wtrocki]

I think api files can easily be moved to the tags but for models that will cause number of issues:
Models reused across the packages will likely need to be redeclared or declared using reusable package

Fix here should enable api files to be saved in the separate folder with subfolders for tag names (property based)