modules/{output,files,test}: move outputs to build scope

Move the following output options into `build`:
- finalPackage -> package
- printInitPackage
- initPath -> initFile
- filesPlugin -> extraFiles
- test.derivation -> test

modules/top-level/test.nix

@@ -36,9 +36,10 @@ in
       description = "Whether to check `config.assertions` in the test.";
       default = true;
     };
+  };
 
-    # Output
-    derivation = lib.mkOption {
+  options.build = {
+    test = lib.mkOption {
       type = lib.types.package;
       description = ''
         A derivation that tests the config by running neovim.
@@ -66,10 +67,10 @@ in
       ) "" toCheck;
     in
     {
-      test.derivation =
+      build.test =
         pkgs.runCommandNoCCLocal cfg.name
           {
-            nativeBuildInputs = [ config.finalPackage ];
+            nativeBuildInputs = [ config.build.package ];
 
             # Allow inspecting the test's module a little from the repl
             # e.g.