The goal is to reduce the flake inputs visible to the user
Still, the top-level flake re-exposes the outputs of the dev-flake, but
without exposing its inputs. This means a devShell is still available in
the top-level, for example.
This also removes the /modules/flake.nix. Its original purpose was
separating the modules inputs from the development inputs, but this is
now done the opposite way around by moving the dev inputs to
/dev-flake/flake.nix.
