feat!(flake): convert to fs-defined configurations

Place all configurations in `machines/${manager}/${system}/${hostname}/...`:
    - `${manager}` - One of `nixos`, `nix-on-droid`, `nix-darwin` or `home-manager`
    - `${system}` - A system's architecture (can be many)
    - `${hostname}` - A system's hostname (can be many)

The flake now automatically generates the needed configurations based on the above structure.

It only generates configurations for valid directory structures:
    - It wouldn't generate a NixOS config if there isn't a `configuration.nix` file
    - ...

lib/default.nix

@@ -0,0 +1,36 @@
+{ lib, ... }:
+
+let
+  inherit (lib) mapAttrs;
+  inherit (lib.attrsets) filterAttrs;
+in
+rec {
+  recurseDir = dir:
+    mapAttrs
+      (file: type:
+        if type == "directory"
+          then recurseDir "${dir}/${file}"
+          else type
+      )
+      (builtins.readDir dir);
+
+  # VVV - Implying `attrs` is the output of `recurseDir` - VVV
+
+  hasFiles = files: attrs:
+    builtins.all
+      (b: b)
+      (builtins.map
+        (file:
+          builtins.hasAttr file attrs &&
+          builtins.getAttr file attrs == "regular")
+        files);
+
+  hasDirectories = directories: attrs:
+    builtins.all
+      (b: b)
+      (builtins.map
+        (directory:
+          builtins.hasAttr directory attrs &&
+          builtins.getAttr directory attrs == "set")
+        directories);
+}