Introduce a new overrideInputs on {default,shell}Nix

[ElvishJerricco]

Porting NixOS#49 with some edits to avoid re-indentation and to fix a null reference bug.

This allows the user of this non-flake to have similar functionality to follows.

Example (default.nix):

nix-repl> (import ./.).default
«derivation /nix/store/axgvq22kyb8ymchzq1mjayms4jdl6ni6-ledc-0.1.0.drv»

nix-repl> ((import ./.).overrideInputs { nixpkgs = <nixpkgs>; }).default
«derivation /nix/store/r8zw693hpg91yx6f57hyx1gk4zdiq2lm-ledc-0.1.0.drv»

[DavHau]

I tested this and it seems to work fine

[alois31]

This breaks on the dwarffs flake (https://github.com/edolstra/dwarffs/tree/9861d09c7a22f86d44e41d5ac049e7b4a8d1998d) with an "expected a string but found a list" error, presumably due to the structure of the nixpkgs follow.

[DavHau]

Suggested change

	rootOverrides =
	mapAttrs'
	(input: lockKey: nameValuePair lockKey (impureOverrides.${input} or null))
	lockFile.nodes.${lockFile.root}.inputs;
	rootOverrides =
	mapAttrs'
	(input: lockKey': let
	lockKey =
	if builtins.isList lockKey'
	then builtins.concatStringsSep "/" lockKey'
	else lockKey';
	in
	nameValuePair lockKey (impureOverrides.${input} or null))
	lockFile.nodes.${lockFile.root}.inputs;

[zimbatm]

For some reason this PR skipped my attention.. what is the status of it?

[ElvishJerricco]

@zimbatm If I understand correctly, @DavHau's suggestion will fix the issue mentioned @alois31, which is that doing something like inputs.nixpkgs.follows = "nix/nixpkgs"; doesn't work with this. I haven't reviewed that change yet to include it. Feel free to do so on my behalf if you'd like to merge this.