Better support for modules in java

The Java implementation does not yet support some module specific logic, such as converting a set of module files to a full model and back.

For reference, the main code that handles modules are:

• Go:
  • https://github.com/openfga/language/blob/main/pkg/go/transformer/module-to-model.go
  • https://github.com/openfga/language/blob/main/pkg/go/transformer/mod-to-json.go
  • test runners:
    • https://github.com/openfga/language/blob/main/pkg/go/transformer/mod-to-json_test.go
    • https://github.com/openfga/language/blob/main/pkg/go/transformer/module-to-model_test.go
• JS:
  • https://github.com/openfga/language/blob/main/pkg/js/util/model_utils.ts#L10-L23 (given a model with modules, give me the list of modules)
  • Tests for it: https://github.com/openfga/language/blob/main/pkg/js/util/model_utils.test.ts#L6-L69
  • https://github.com/openfga/language/tree/main/pkg/js/transformer/modules
  • test runners: https://github.com/openfga/language/tree/main/pkg/js/tests/modules

The tests are shared across all packages, and only the test runner needs to be implemented. Test data specific to modules

• https://github.com/openfga/language/blob/main/tests/data/fga-mod-transformer-cases.yaml
• https://github.com/openfga/language/tree/main/tests/data/transformer-module

Start by comparing the referenced Go and JavaScript module transformers and their test runners, then inspect the shared cases in tests/data/fga-mod-transformer-cases.yaml and tests/data/transformer-module. The work is complete when the Java implementation converts module files to a full model and back, and the module cases pass through its test runner.