Make the library a proper JPMS module

Right now, when we add:

```java
module com.example {
requires anthropic.java.core;
requires anthropic.java.client.okhttp;
// etc.
}
```

to our project's `module-info.java` and compile the project with Maven, we get a warning:

```
[WARNING] * Required filename-based automodules detected: [anthropic-java-client-okhttp-2.48.0.jar, anthropic-java-core-2.48.0.jar]. Please don't publish this project to a public artifact repository! *
```