Add flakeModules.easyOverlay code

dev/tests/eval-tests.nix

@@ -1,5 +1,10 @@
 rec {
   f-p = builtins.getFlake (toString ../..);
+  flake-parts = f-p;
+
+  devFlake = builtins.getFlake (toString ../.);
+  nixpkgs = devFlake.inputs.nixpkgs;
+
   f-p-lib = f-p.lib;
 
   inherit (f-p-lib) mkFlake;
@@ -26,6 +31,35 @@ rec {
       };
     };
 
+  easyOverlay = mkFlake
+    { self = { }; }
+    {
+      imports = [ flake-parts.flakeModules.easyOverlay ];
+      systems = [ "a" ];
+      perSystem = { system, config, final, pkgs, ... }: {
+        packages.default = config.packages.hello;
+        packages.hello = pkg system "hello";
+        packages.hello_new = final.hello;
+        overlayAttrs = {
+          hello = config.packages.hello;
+          hello_old = pkgs.hello;
+          hello_new = config.packages.hello_new;
+        };
+      };
+    };
+
+  nixpkgsWithoutEasyOverlay = import nixpkgs {
+    system = "x86_64-linux";
+    overlays = [ ];
+    config = { };
+  };
+
+  nixpkgsWithEasyOverlay = import nixpkgs {
+    system = "x86_64-linux";
+    overlays = [ easyOverlay.overlays.default ];
+    config = { };
+  };
+
   runTests = ok:
 
     assert empty == {
@@ -55,6 +89,19 @@ rec {
       };
     };
 
+    # - exported package becomes part of overlay.
+    # - perSystem is invoked for the right system.
+    assert nixpkgsWithEasyOverlay.hello == pkg "x86_64-linux" "hello";
+
+    # - Non-exported package does not become part of overlay.
+    assert nixpkgsWithEasyOverlay.default or null != pkg "x86_64-linux" "hello";
+
+    # - hello_old comes from super
+    assert nixpkgsWithEasyOverlay.hello_old == nixpkgsWithoutEasyOverlay.hello;
+
+    # - `hello_new` shows that the `final` wiring works
+    assert nixpkgsWithEasyOverlay.hello_new == nixpkgsWithEasyOverlay.hello;
+
     ok;
 
   result = runTests "ok";