Support custom python libraries in dev shell nixland

[drbh]

This PR supersedes #150 and avoids adding the extra libraries into the build.toml.

example flake

{
  description = "Flake for megablocks_moe kernel";
  
  inputs = {
    kernel-builder.url = "github:huggingface/kernel-builder/support-custom-python-libraries-in-dev-shell-nixland";
    # Add libraries as inputs
    composer = {
      url = "github:mosaicml/composer";
      flake = false;
    };
    stk = {
      url = "github:stanford-futuredata/stk";
      flake = false;
    };
  };
  
  outputs = {
    self,
    kernel-builder,
    composer,
    stk,
  }:
    kernel-builder.lib.genFlakeOutputs {
      path = ./.;
      rev = self.shortRev or self.dirtyShortRev or self.lastModifiedDate;
      
      # Map custom packages to their sources
      customPythonPackages = {
        composer = composer;
        stk = stk;
      };
      
      pythonTestDeps = [
        "tqdm"
        "py-cpuinfo"
        "importlib-metadata"
        "torchmetrics"
        "composer"
        "stk" 
      ];
    };
}

Note: I was unclear on how to best include the custom Python package in an idiomatic Nix way and settled on the current solution. If there is an easier way to resolve Git repositories as libraries, that would be preferred.

[danieldk]

I think this can be closed in favor of #155?