Fix import path.

Trying to create a simple flake with input:

```
{
inputs.nixpkgs-lib.url = "github:NixOS/nixpkgs?dir=lib";
outputs = {nixpkgs-lib, ...}: { };
}
```

Fails when trying to evaluate the nixpkgs-lib flake with an error like:

```
error: getting status of '/nix/store/xxxx-source/lib/lib': No such file or directory
(use '--show-trace' to show detailed location information)
```

lib/flake.nix

···
 {
   description = "Library of low-level helper functions for nix expressions.";
 
-  outputs = { self }: { lib = import ./lib; };
+  outputs = { self }: { lib = import ./.; };
 }