After modifying the source file, the corresponding automatically generated class is not updated

### Questions
After modifying the source file, the corresponding automatically generated class is not updated.

### Version

4.0.0-milestone4

### Context

while I used vertx to automatically generate classes such as VertxProxyHandler and VertxEBProxy, I encountered a problem, that is:
when I modify a certain service source code file, the IDE will automatically recompile it, but the corresponding VertxProxyHandler and VertxEBProxy classes have not been recompile.

After debugging, I found that it was caused by this line of code in the process method of the CodeGenProcessor class in the vertx-codgen class library.


As long as classes such as VertxProxyHandler are automatically generated once, unless the entire project is recompiled as a whole or the generated class is deleted, it will not be automatically generated in the future.

### Do you have a reproducer?

https://github.com/changchengqin/codegen

### Steps to reproduce

1. clone above repo.
2. build
3. modify source file `src/main/java/com/example/codegen/service/UserService.java`
4. recompile source file `src/main/java/com/example/codegen/service/UserService.java`

The normal expected result is that UserServiceVertxProxyHandler and UserServiceEBProxy should be updated accordingly, but the fact is not.
### Extra

* OS: Windows 10 Enterprise
* JDK: 11 or later
* IDE: Intellij IDEA Community 2019.3.1 or 2019.3.3 or 2019.3.4 or 2020.2

レポートで参照されているCodeGenProcessorのprocessメソッドから始め、リンクされたcodegenプロジェクトを使用して動作を再現します。src/main/java/com/example/codegen/service/UserService.javaを変更して再コンパイルし、UserServiceVertxProxyHandlerとUserServiceEBProxyが再生成されるか確認します。両方の生成クラスにソースの変更が反映されれば完了です。

索引モデルが issue の本文から書いたものです。