Fix for getting core_path

[fcollonval]

@cronan03 to solve the issue for

https://github.com/jupyterlab/jupyterlab-builder/blob/727dc285ea97b730068731c177915144b61a8fd7/jupyter_builder/federated_extensions.py#L202

I suggest you replace osp.join(HERE, "staging") by a call to a new function default_core_path that will have the following implementation:

from pathlib import Path

def default_core_path() -> str:
    import jupyterlab
    return str(Path(jupyterlab.__file__).parent / "staging")

[cronan03]

Hey, this was the error I was facing.