feat: fall back to loading module package when module file not found

Extract file path construction into `wrenFilePath` helper and modify `readModule` to first attempt loading the module as a `.wren` file. If that fails, treat the module name as a package directory and try to load `module.wren` from within it. Add test files `module_package/module.wren` and `package_module.wren` to verify the new package resolution behavior.

test/language/module/module_package/module.wren

@@ -0,0 +1 @@
+var Index = "index"

test/language/module/package_module.wren

@@ -0,0 +1,3 @@
+import "module_package" for Index
+
+IO.print(Index) // expect: index