[Feature Request] Allow adding a package from file system

[amok]

It'd be nice to be able to install packages from nix files from local FS.

F.i.:

devbox add /path/to/default.nix
devbox add path/to/package

---

Usecases

Let say you have a small dev proxy server my-development-proxy-server written in node and It's not part of the main app. my-development-proxy-server/default.nix would be something like

# This is a minimal `default.nix` by yarn-plugin-nixify. You can customize it
# as needed, it will not be overwritten by the plugin.

{ pkgs ? import <nixpkgs> { } }:

pkgs.callPackage ./yarn-project.nix { } { src = ./.; }

Then shell.nix would look something like

let
  pkgs = import <nixpkgs> { };
  myDevelopemntProxyServer = import ./my-development-proxy-server;
  python = pkgs.python38;
  .....
in
pkgs.mkShell {
  packages = [
		python
		myDevelopemntProxyServer
		....
  ];
}

For the time being as far as I know it's not possible to devbox add ./my-development-proxy-server

Another use case is overriding package attrs. Is there a way to implement the following with devbox?:

let
  pkgs = import <nixpkgs> { };
  nginx = pkgs.nginx.override { modules = [ pkgs.nginxModules.njs ]; };
  nodejs = pkgs.nodejs-slim-14_x;
  yarn = pkgs.yarn.override { inherit nodejs; };
  ...

As an option, we could have yarn.nix like:

let
  pkgs = import <nixpkgs> { };
  nodejs = pkgs.nodejs-slim-14_x;
in
pkgs.yarn.override { inherit nodejs; }

and do devbox add ./yarn.nix

---

Unfortunately, this blocks me switching from "raw" nix to devbox (the tool looks very exciting)
Let me please know if I am missing something.

Similar issues: #267 #117 #21

PS Thank you for the awesome tool

[Lagoja]

Hey @amok, thanks for filing this issue. We really love the idea and have been discussing it within our team, since this could be a great bridge for existing nix projects to take advantage of Devbox.

We're reviewing the idea internally to see if we can slot it for an upcoming release.

[Lagoja]

See #866, which will allow developers to add local flakes as a source. Would be great to get your thoughts on what else to include.