[BUG][JAVA] Unable to compile newly added generator code

Created by: godrose

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

openapi-generator version

Version: 4.2.3

Steps to repro

Follow the steps outlined at https://openapi-generator.tech/docs/customization.html

Run 'Use the new generator' step with the following command java -cp out/generators/my-codegen/target/my-codegen-openapi-generator-1.0.0.jar:modules/openapi-generator-cli/target/openapi-generator-cli.jar org.openapitools.codegen.OpenAPIGenerator

Expected: It works Actual: It doesn't. The error is the following: Error: Could not find or load main class org.openapitools.codegen.OpenAPIGenerator