unify overlays

We used to have overlay definitions in home/overlays and
packages/default. Let's unify them.
Using the exposed overlay from the flake required some changes in the
home-manager config though.

packages/default.nix

@@ -5,4 +5,32 @@ final: prev: {
       spsdk = python-prev.spsdk.overridePythonAttrs { doCheck = false; };
     };
   };
+  fc-scripts = final.callPackage ./fc-scripts.nix { };
+  myVim = prev.neovim.override {
+    configure = {
+      packages.myVimPackage = with final.pkgs.vimPlugins; {
+        # loaded on start
+        start = [
+          ale
+          base16-vim
+          LanguageClient-neovim
+          ctrlp
+          direnv-vim
+          fugitive
+          haskell-vim
+          jedi-vim
+          nerdtree
+          nerdtree-git-plugin
+          supertab
+          vim-dirdiff
+          vim-pandoc
+          vim-pandoc-syntax
+          vim-nix
+          vim-jinja
+          which-key-nvim
+        ];
+      };
+      customRC = builtins.readFile ../home/vimrc;
+    };
+  };
 }