Submodules appear as regular directories

```
core.workflow(
name = "default",
origin = git.origin(
...
submodules = "RECURSIVE",
...
),
destination = git.destination(
...
),

destination_files = glob(["**"]),
origin_files = glob(["**"]),

authoring = authoring.pass_thru("Email"),
)
```
I have a module having various submodules in it. I am using copybara to copybara non IP code to the public repository. However, when I run the tool, my submodule gets copied as a normal directory rather than getting copied as submodule.