feat: add fs.get-directories-with-default helper to eliminate duplicate directory filtering

snowfall-lib/fs/default.nix

@@ -99,6 +99,20 @@ in
       in
       mapAttrsToList (name: kind: "${path}/${name}") filtered-entries;
 
+    ## Get directories containing default.nix at a given path.
+    ## Example Usage:
+    ## ```nix
+    ## get-directories-with-default ./systems/x86_64-linux
+    ## ```
+    ## Result:
+    ## ```nix
+    ## [ "./systems/x86_64-linux/my-host" ]
+    ## ```
+    #@ Path -> [Path]
+    get-directories-with-default =
+      path:
+      builtins.filter (dir: pathExists "${dir}/default.nix") (get-directories path);
+
     ## Get files at a given path.
     ## Example Usage:
     ## ```nix

snowfall-lib/home/default.nix

@@ -179,8 +179,7 @@ in
     get-target-homes-metadata =
       target:
       let
-        homes = snowfall-lib.fs.get-directories target;
-        existing-homes = builtins.filter (home: builtins.pathExists "${home}/default.nix") homes;
+        existing-homes = snowfall-lib.fs.get-directories-with-default target;
         create-home-metadata =
           path:
           let

snowfall-lib/system/default.nix

@@ -106,8 +106,7 @@ in
     get-target-systems-metadata =
       target:
       let
-        systems = snowfall-lib.fs.get-directories target;
-        existing-systems = builtins.filter (system: builtins.pathExists "${system}/default.nix") systems;
+        existing-systems = snowfall-lib.fs.get-directories-with-default target;
         create-system-metadata = path: {
           path = "${path}/default.nix";
           # We are building flake outputs based on file contents. Nix doesn't like this