Support flake in a subdir of `src`

[deemp]

Context

I have a project where I have two flakes:

• ./flake.nix
• ./nix-dev/flake.nix

./flake.nix uses inputs from ./nix-dev/flake.nix:

• https://github.com/nix-community/nix-vscode-extensions/blob/842ee6c0a54cd4d0ed53ab5c8df88368cd1f382f/flake.nix#L25-L31
• https://github.com/nix-community/nix-vscode-extensions/blob/842ee6c0a54cd4d0ed53ab5c8df88368cd1f382f/flake.nix#L37

./nix-dev/flake.nix accesses files in ./ via the path:../ input:

• https://github.com/nix-community/nix-vscode-extensions/blob/842ee6c0a54cd4d0ed53ab5c8df88368cd1f382f/nix-dev/flake.nix#L35-L38
• https://github.com/nix-community/nix-vscode-extensions/blob/842ee6c0a54cd4d0ed53ab5c8df88368cd1f382f/nix-dev/flake.nix#L44

Problem

In the current flake-compat implementation, the src coincides with rootSrc.

Therefore, if I set src to ./nix-dev, I get an error

 at /nix/store/ndslvddkj5h37rn09ccnk9s8bzydcz3z-nix-dev/flake.nix:44:24:
           43|       parent = inputs.parent.outPath;
           44|       systemPlatform = import "${parent}/nix/systemPlatform.nix";
             |                        ^
           45|       systems = builtins.attrNames systemPlatform;

       error: access to absolute path '/nix/store/nix/systemPlatform.nix' is forbidden in pure evaluation mode (use '--impure' to override)

Solution

Introduce an additional argument to the main function in default.nix that will let a user specify the "root" within src, i.e. where the flake.nix is within src.

This suggestion is implemented in #90.