[gazelle] Allow other plugins to also contribute to class location

[shs96c]

When multiple rules create a "split package", Gazelle needs class-level resolution to determine which rule provides which class. Previously, only the built-in Java plugin could participate in this resolution via the internal classExportCache. Other gazelle plugins which generate java classes (such as a possible one for Wire) could not.

This PR introduces a SharedClassCache in config.Exts that external plugins can populate with their class information:

// In external plugin's GenerateRules:
cache := javaconfig.GetOrCreateSharedClassCache(args.Config)
cache["//pkg:my_wire_rule"] = javaconfig.SharedClassInfo{
    Classes:  []string{"com.example.MyGeneratedClass"},
    TestOnly: false,
}

The Java plugin's buildPackageClassIndex now consults both:

1. Internal classExportCache (for java_library rules)
2. SharedClassCache (for external plugin rules)