Hosts, Use directory-driven hosts in flake.nix

This adds some code to flake.nix to read the directory of hosts/ and
find all hosts based on their host.nix file, and set the hostname based
on the directory.

I don't know enough about hostnames to know what are valid characters,
but in the future this could allow a hostname such as "servers/madoka"
which may be an issue. For now the structure is simple enough of
"hosts/name" resulting in hostname "name"

Another commit is planned which will do the same thing for users

flake.nix

@@ -15,6 +15,8 @@
   outputs = { self, home-manager, nixpkgs, ... }@inputs: let
     system = "x86_64-linux";
     pkgs = nixpkgs.legacyPackages.${system};
+    fs = pkgs.lib.fileset;
+    st = pkgs.lib.strings;
 
     hostConfig = extraModules: nixpkgs.lib.nixosSystem {
       specialArgs = { inherit inputs; };
@@ -23,6 +25,15 @@
         ./nixosModules
       ] ++ extraModules;
     };
+    hostFilter = { name, ...}: name == "host.nix";
+    hostPaths = fs.toList (fs.fileFilter hostFilter ./hosts);
+
+    hosts = builtins.listToAttrs (map (path: {
+      value = hostConfig [ path ];
+      name = builtins.unsafeDiscardStringContext (st.removeSuffix "/host.nix" (
+        builtins.elemAt (st.splitString "/hosts/" path) 1
+      ));
+    }) hostPaths);
 
     userConfig = extraModules: home-manager.lib.homeManagerConfiguration {
       inherit pkgs;
@@ -31,12 +42,11 @@
         ./hmModules
       ] ++ extraModules;
     };
-  in
-  {
-    nixosConfigurations.onizuka = hostConfig [ ./hosts/onizuka ];
-    homeConfigurations."pan@onizuka" = userConfig [ ./hosts/onizuka/users/pan ];
 
-    nixosConfigurations.jibril = hostConfig [ ./hosts/jibril ];
+  in {
+    nixosConfigurations = hosts;
+
+    homeConfigurations."pan@onizuka" = userConfig [ ./hosts/onizuka/users/pan ];
     homeConfigurations."pan@jibril" = userConfig [ ./hosts/jibril/users/pan ];
   };
 }