Flake support

[edolstra]

This PR adds experimental support for flakes. The flake file format is defined by RFC 49. To use it, you have to add

experimental-features = nix-command flakes

to nix.conf.

Other changes:

• The nix command is flake-based now. For instance, nix build nixpkgs#hello builds the hello package from the nixpkgs flake. Non-flake-based operation is still supported, e.g. nix build -f /path/to/expr.nix. However, nix no longer uses NIX_PATH so it's no longer affected by channels.

• New subcommand nix flake for working with flakes.

• New subcommand nix profile for imperative package management (intended to replace nix-env). (This is not completely finished. Note that once a profile has been modified using nix profile, you can't use nix-env on it anymore.)

• New subcommand nix run (previously named nix app) to run an application defined by an app or defaultApp output of a flake.

• The nix command has an evaluation cache to speed up querying flake attributes. For instance, nix search uses this instead of the old JSON cache.

[mkenigs]

I've been thinking about usability, and I guess some of this might be more of my thoughts related to nix in general. But if I was trying to to convince someone to use nix, I would want a flake to look something more like this:

{
  description = "optional description";
  inputs.build = [ boost ];
  inputs.test = [ testframework ];
  outputs = { self, inputs }: {
    mkFlake {
      name = "name";
      build =
        ''
          make
        '';
      install =
        ''
          cp name $out/bin/
        '';
      run = "$out/bin/name";
      test =
        ''
          test
        '';
    }
  }
}

I don't know how things like nixpkgs, system, mkDerivation, and other outputs would be handled, so I guess this is more just my thoughts rather than something concrete. But if it was possible, I think something along these lines would be way easier to digest as a newcomer to nix. It's pretty easy to understand what's going on: you say what dependencies you need, nix does it's nix magic, and you get something you can build/run/develop/test anywhere with nix build/run/develop/test (flake check). I think it's hard to realize that's what nix can do when it's hidden behind complexity, but it's actually incredibly appealing once you see it.

[edolstra]

@mkenigs Agree completely but that's out of scope for the first iteration of flakes. In the future we might have a flake.toml containing a high-level description of a package, without any extraneous cruft like mkDerivation, function calls etc.