make it possible to place models in a different package to prevent circular package references for go-server

[ben-sab]

the changes have been tested with a few projects including the pet-store project:
java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate --input-spec https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/3_0/petstore.yaml --model-package models/ --generator-name go-server --output generated

related issues:

• [REQ] [go-gin-server] place models in different package to prevent circular package references  #2752 (although this issue is about go-gin-server, changes to make go-gin-server work as well would be minimal)
• generator ignore package passed for model and API for go generator #2803
• [REQ] Better Structure for Go Server Project #1931

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[wing328]

Does go client generator have similar issues?

[ben-sab]

@wing328 I haven't actually tested the generated client properly yet, but yes after generating the client code I can see that the --model-package flag isn't taken into account. I can make a separate PR for that if this gets approved.

[wtrocki]

Small rebase is needed. I'm not maintainer but I have tested this change on client side already.
Without this change using some complex openapis fails

[wtrocki]

Amazing changes. Crossing fingers for this to land soon. It will be game changer for golang ecosystem to have this done