Separate CoreML util and inmemoryfs targets to be reusable (#9481)

### Summary
In this diff, we separate the targets for `coreml/runtime/util` and
`coreml/runtime/inmemoryfs`.

#### Why?

As part of https://github.com/pytorch/executorch/issues/9019, we want to
support CoreML export out-of-the-box. Unfortunately, part of the
workflow now includes installing an executorch+coreml binding as a pip
wheel:

https://github.com/pytorch/executorch/blob/5fdfa511966208c7e237a9e920a7c63f513b4fb7/backends/apple/coreml/scripts/install_inmemoryfs.sh#L20

This then gets used throughout the library. i.e.


https://github.com/pytorch/executorch/blob/5fdfa511966208c7e237a9e920a7c63f513b4fb7/backends/apple/coreml/compiler/coreml_preprocess.py#L19

So we want to now bake this pybinding into the main wheel. Ultimately
this `executorchcoreml` module, gets built using inmemoryfs:


https://github.com/pytorch/executorch/blob/5fdfa511966208c7e237a9e920a7c63f513b4fb7/backends/apple/coreml/runtime/inmemoryfs/setup.py#L17-L38

So, to enable creating a pybinding target, we must first decouple the
`util` and `inmemoryfs` libraries so they can be used in
`coremldelegate` and the future pybinding.

### Test plan
This should really be a no-op w.r.t. builds. So, CI +
```
$ ./install_executorch.sh --pybind coreml
```


cc @larryliu0820 @lucylq