Refactor imports and secrets. Add per system properties and role based secret access.

Highlights
- No need to update flake for every machine anymore, just add a properties.nix file.
- Roles are automatically generated from all machine configurations.
- Roles and their secrets automatically are grouped and show up in agenix secrets.nix
- Machines and their service configs may now query the properties of all machines.
- Machine configuration and secrets are now competely isolated into each machine's directory.
- Safety checks to ensure no mixing of luks unlocking secrets and hosts with primary ones.
- SSH pubkeys no longer centrally stored but instead per machine where the private key lies for better cleanup.

machines/phil/default.nix

@@ -0,0 +1,9 @@
+{ config, pkgs, lib, ... }:
+
+{
+  imports = [
+    ./hardware-configuration.nix
+  ];
+
+  networking.hostName = "phil";
+}