empack_config file filter does not seem to be working

[katotetsuro]

Description

The file filter specified in XeusAddon.empack_config is not used.

https://github.com/jupyterlite/xeus/blob/main/jupyterlite_xeus/add_on.py#L263-L287

Reproduce

jupyter lite build --XeusAddon.empack_config=empack_config.yml

Context

It might be better handled as follows

https://github.com/jupyterlite/xeus-python-kernel/blob/main/jupyterlite_xeus_python/build.py#L321

pack_env(
      env_prefix=self.prefix,
      relocate_prefix="/",
      outdir=out_path,
      use_cache=True,
      **pack_kwargs,
)

[martinRenou]

Thank you for opening an issue. I blame myself for not testing the feature properly.

Would you like to open a PR to get credits for your fix?

[katotetsuro]

I would like to!
I have created a PR here.
#53