fix: only show flake and dir warning when relevant

Forgetting to set `src` was triggering it.
Ideally we'd just change the function to use `{ src, ... }@Args:` so Nix
can provide an explicit error the users are accustomed to.

Author: ThinkChaos

nix/load.nix

@@ -30,9 +30,14 @@ args:
 
 let
   src = toString (
-    args.src or (warn
-      "namaka.load: `flake` and `dir` have been deprecated, use `src` directly instead"
-      (args.flake + "/${args.dir or "tests"}"))
+    if args ? src then
+      args.src
+    else if args ? flake then
+      warn
+        "namaka.load: `flake` and `dir` have been deprecated, use `src` directly instead"
+        (args.flake + "/${args.dir or "tests"}")
+    else
+      throw "namaka.load: missing mandatory `src' argument"
   );
 
   tests = haumea.load (removeAttrs args [ "flake" "dir" ] // {