Fix the segfault on nix profile install with conflict

[thufschmitt]

Motivation

nix profile install is currently segfaulting if

1. There's a conflict between the installables, and
2. One of these installables doesn't have a known flakeref

Besides, the error message shows the original flakeref but without the fragment part, meaning that it is wrong if the installable comes from a flake but isn't the default package of said flake.

Fix that by:

• If the element comes from a flake, print the full flakeref (with the fragment part) and not just the reference to the flake itself
• If the element doesn't come from a flake, print its store path(s)

This is a bit too verbose, but has the advantages of being correct (and not crashing), so it's strictly better than the previous situation

Context

Fix #8284

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[dermetfan]

I can confirm that this fixes the issue:

(setup)

❯ cat flake.nix
{
  inputs.nixpkgs.url = github:nixos/nixpkgs/nixos-22.11;

  outputs = { nixpkgs, ... }: {
    packages.x86_64-linux = let
      foo = name: nixpkgs.legacyPackages.x86_64-linux.runCommand "foo-${name}" {} ''
        mkdir $out
        echo ${nixpkgs.lib.escapeShellArg name} > $out/foo
      '';
    in {
      a = foo "a";
      b = foo "b";
    };
  };
}
❯ nix build .#a -o a
❯ nix build .#b -o b

❯ nix run github:nixos/nix/2.15-maintenance -- profile install --profile ./profile ./{a,b}
fish: Job 1, 'command nix $argv' terminated by signal SIGSEGV (Address boundary error)

❯ nix run github:tweag/nix/fix-nix-profile-install-conflict-segfault -- profile install --profile ./profile ./{a,b}
error: An existing package already provides the following file:

         /nix/store/s5z281h1xn24wzfwmdr6699lbdc4xbdi-foo-a/foo

       This is the conflicting file from the new package:

         /nix/store/7dlry4dhs100izx3mdjvbvmzq7mkdfgj-foo-b/foo

       To remove the existing package:

         nix profile remove /nix/store/s5z281h1xn24wzfwmdr6699lbdc4xbdi-foo-a

       The new package can also be installed next to the existing one by assigning a different priority.
       The conflicting packages have a priority of 5.
       To prioritise the new package:

         nix profile install /nix/store/7dlry4dhs100izx3mdjvbvmzq7mkdfgj-foo-b --priority 4

       To prioritise the existing package:

         nix profile install /nix/store/7dlry4dhs100izx3mdjvbvmzq7mkdfgj-foo-b --priority 6

[Ericson2314]

Thanks!