Separate `overlay.nix` and `overlay.internal.nix`, to encapsulate the
`flake` argument. This allows non-flake users to import `./overlay.nix`
without providing a dummy `flake = null` attr.
The read-only `config.flake` option is only defined by `evalNixvim` when
`lib` is extended using the overlay in Nixvim's flake outputs.