fix: use default.nix to inject inputsOverrides if available

[jbgi]

using the flake api breaks when flakes use the following pattern:

{
    outputs = args: let f = { self }: ...; in f args;
}

(which is the case of nixpkgs-lib).

(PR based on #21 to avoid rebase)

[jbgi]

@vic fixed a remaining issue in last commit, mostly for the cases where one use a with-inputs project as a flake project (eg. for template init): trying to import a default.nix input that use builtins.currentSystem was failing (eg. when it is implemented with flake-compat), so modified logic to don't try that if builtins.currentSystem is not present. Also to don't to try to import default.nix if flake.nix actually define inputs specs (more of an optimization to avoid unecessary import of default.nix).