Generate OpenAPI code in output directory

[sakanaou]

While integrating this project in my example at https://github.com/sakanaou/openapi-first I noticed that the OpenAPI code is generated in a sub-directory of the actual project and not in the output directory of Bazel. Therefore the code is not properly cleaned and the sub-directory would need to be excluded via .gitignore. I changed the Bazel script so that the OpenAPI code is generated in the proper output directory, solving both mentioned issues.