[flakes] add and remove packages, individually

[savil]

Summary

This PR adds the following top-level functions:

• devbox.addPackagesToProfile: used during devbox add and devbox shell.
• devbox.removePackagesFromProfile: used during devbox rm.

This PR's size comes from inspecting the results of nix profile list, which we parse into nix.ProfileListItem structs. We use this information in two ways:

1. In devbox.addPackagesToProfile, we inspect list of nix.ProfileListItem to determine which of the devbox.json packages are pending installation.
2. In devbox.removePackagesFromProfile, we need the "attribute path" of the package, which we derive from its nix.ProfileListItem

UX:
The UX can be improved, but I didn't invest in it since the PR size is already large. That can be a discussion and a follow up PR.

TODOs for future PRs:

• Ensure the github:NixOS/nixpkgs/<commit> URL can leverage nixed cache service
• During devbox add we always list nixpkgs as being installed, which is kinda odd. Can we check that it has been pre-fetched already?
• UX improvements
• Older non-flakes profiles link to bin, lib, share, but with flakes profiles we just link to bin and share

How was it tested?

>  DEVBOX_FEATURE_FLAKES=1 DEVBOX_FEATURE_NIXLESS_SHELL=1 DEVBOX_DEBUG=0 devbox shell
(devbox)>  DEVBOX_FEATURE_FLAKES=1 DEVBOX_FEATURE_NIXLESS_SHELL=1 DEVBOX_DEBUG=0 devbox add vim emacs
(devbox)>  DEVBOX_FEATURE_FLAKES=1 DEVBOX_FEATURE_NIXLESS_SHELL=1 DEVBOX_DEBUG=0 devbox rm emacs

Installing looks like below. I did not use the stepper component so that we show the output of each individual install.

[savil]

Current dependencies on/for this PR:

• main
  • PR [flakes | print-dev-env] enable flakes to work with print-dev-env #584
    • PR [flakes] add and remove packages, individually #597 👈

This comment was auto-generated by Graphite.

[savil]

I think this and below code is better moved to nix/profile.go so it'll be nix.ProfileListItems()

[gcurtis]

I think package name collisions are the only must-fix thing. Is there a performance difference in invoking Nix for each package vs. doing them all at once?

[gcurtis]

A potential way to simplify the parsing is to use a bufio.Scanner. For example:

out, err := cmd.StdoutPipe()
scanner := bufio.NewScanner(out)
scanner.Split(bufio.ScanWords)
for scanner.Scan() {
    index, err := strconv.Atoi(scanner.Text())
    if !scanner.Scan() {
        return errors.New("incomplete nix profile list line")
    }
    unlockedReference := scanner.Text()
    if !scanner.Scan() {
        return errors.New("incomplete nix profile list line")
    }
    lockedReference := scanner.Text()
    if !scanner.Scan() {
        return errors.New("incomplete nix profile list line")
    }
    nixStorePath := scanner.Text()

    items = append(items, ...)
}

[savil]

Much nicer now, thanks for the tip to use scanner (and helpful code).

[gcurtis]

This won't necessarily be unique. For example, python310Packages.numpy and python311Packages.numpy will both become numpy. I think that'll affect the maps that use the package name as a key.

[savil]

Good point.

I could parse it the other direction assuming the prefix will always be legacyPackages.<platform>

[savil]

Added a test case and updated code to fix this.