[BUG] [python-fastapi] fastapiImplementationPackage ignores packageName option in favor of the "openapi_server.impl" default package name

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?
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Description

When running the openapi-generator with the argument

All generated apis Python files will import

import openapi_server.impl

instead of the expected

import my_package.impl

I did not provide an example swagger.yml file as this is a general problem for arbitrary files.

openapi-generator version

v7.9.0

Generation Details

generate -i swagger.yml \
                  -g python-fastapi \
	          -o out \
	          --additional-properties=packageName=my_package

Suggest a fix

I think the following line within modules\openapi-generator\src\main\java\org\openapitools\codegen\languages\PythonFastAPIServerCodegen.java could fix for that (or the documentation of python-fastapi generator should be adapted):

    @Override
    public void processOpts() {
        super.processOpts();

        if (additionalProperties.containsKey(CodegenConstants.PACKAGE_NAME)) {
            setPackageName((String) additionalProperties.get(CodegenConstants.PACKAGE_NAME));
        }

should read

    @Override
    public void processOpts() {
        super.processOpts();
        if (additionalProperties.containsKey(CodegenConstants.PACKAGE_NAME)) {
            setPackageName((String) additionalProperties.get(CodegenConstants.PACKAGE_NAME));
            this.implPackage = ((String) additionalProperties.get(CodegenConstants.PACKAGE_NAME)).concat(".impl");
        }

Start in modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonFastAPIServerCodegen.java and inspect processOpts(), especially how packageName and implPackage are initialized. Run the supplied python-fastapi CLI command with packageName=my_package and verify that generated API files import my_package.impl instead of openapi_server.impl.