Separate truely top-level code from per-bootstrapping-stage code

[Ericson2314]

Motivation for this change

I think top-level was getting a but unwieldy. With this change:

• Truely top level stays in pkgs/top-level
• Per-bootstrapping-stage goes in pkgs/stage

Things done

Gonna let Travis test this one

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

[mention-bot]

@Ericson2314, thanks for your PR! By analyzing the history of the files in this pull request, we identified @aske, @obadz, @rardiol and @FRidh to be potential reviewers.

[copumpkin]

Renaming files that everyone changes all the time is going to break almost every open PR or branch...

[Ericson2314]

Hmm git itself can easily deal with renamed files, but github isn't so clever.

[FRidh]

Is there an overview of what the desired structure of the top-level functions and repository layout is?

In my opinion we should just move python-packages.nix to pkgs/development/python-modules/default.nix.

[Ericson2314]

@FRidh this is my own vison, if you will :). If you see how I split top-level/default.nix into top-level/stage.nix in #15043, this is moving things around to further that distinction.

[nbp]

These are not stages, but collections / indexes of packages.
The stages would be the steps taken by the top-level to aggregate stdenv and others.

I think pkgs/indexes/node-packages.nix makes more sense than pkgs/stages/node-packages.nix.
Also, if all these indexes / collections are suffixed by -packages, maybe we should move this particle to the directory name, if it is not explicit yet, such as pkgs/indexes/npm.nix, pkgs/indexes/pip.nix, pkgs/indexes/cabal.nix ?

What do you think?

[Ericson2314]

@nbp I like it! While yes stage/default.nix computes a single bootstrapping stage, that involves little more than aggregating the indices. And certainly everything else in there is an index that just happens to be used for computing a single stage—and it's better to label our "nouns" by what they are than how they are used.

I'll change this PR accordingly in a few minites, and also change the documentation in now-called indices/default.nix to philosophicly align.

[Ericson2314]

Not sure if I'm actually causing some sort of pythonic mass rebuild, or Travis was just having a bad day.

[mmahut]

Any update on this pull request?

[FRidh]

I think moving the package sets into their own folder is not a bad idea, although it's a pain when going through the history. if we would do this, then may as well create a new commit and PR.