migrate constructSystem to lib/

migrates the bulk of constructSystem to lib/ and splits out the largest
chunks into individual functions (namely SOPS, home-manager, and user creation);

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

flake.lock

@@ -10,7 +10,9 @@
         "nixpkgs": [
           "nixpkgs"
         ],
-        "nixpkgs-stable": "nixpkgs-stable"
+        "nixpkgs-stable": [
+          "nixpkgs-stable"
+        ]
       },
       "locked": {
         "lastModified": 1711742460,
@@ -458,7 +460,7 @@
         "nixos-hardware": "nixos-hardware",
         "nixos-modules": "nixos-modules",
         "nixpkgs": "nixpkgs",
-        "nixpkgs-stable": "nixpkgs-stable_2",
+        "nixpkgs-stable": "nixpkgs-stable",
         "rust-overlay": "rust-overlay",
         "sops-nix": "sops-nix",
         "systems": "systems",