[nix] Allow github flakes in devbox.json

[mikeland73]

Summary

This change allows packages to be specified like so:

 "packages": [
    "github:nixos/nixpkgs/5233fd2ba76a3accb5aaa999c00509a11fd0793c#hello",
    "github:F1bonacc1/process-compose"
  ],

It works similar to local flakes.

We use nix search to determine the attribute path. If zero or more than 1 are available, we show error.

Other improvements:

• Prevent flakes from being listed more than once in inputs
• Use search instead of show for faster validation of outputs.

TODO:

• Found bug when doing devbox rm. ProfileListIndex not working correctly for defaultPackage

TODO: (in follow up):

• Optimize by caching nix search

How was it tested?

• example
• unit tests

[mikeland73]

@gcurtis this changed a little bit, can you PTAL? Thanks!