fix(turbopack): use var to replace const at esm export codegen

[fireairforce]

主要为了修复: utooland/utoo#2385

有个循环依赖的例子:

// a.ts
import c from './c';

export default c;

// c.ts
import a from './a';

a;

const c = 1;
export default c;

在 codegen 的时候 turbopack 会把 a module 里面的 c module 的导出给提上来，看上去是为了避免 import 消费的模块能找到对应的变量。

参考代码逻辑: https://github.com/utooland/next.js/blob/utoo/turbopack/crates/turbopack-ecmascript/src/references/esm/export.rs#L825

于是就会构造出 issue 中的报错，这里简单的解决办法就是把 const 替换成 var，让变量同样提升上去。

本地测试正常。

[gemini-code-assist]

Summary of Changes

Hello @fireairforce, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

此拉取请求解决了 Turbopack 在 ESM 导出代码生成中，当存在循环依赖时遇到的特定问题。通过将默认导出变量的声明从 const 切换到 var，确保了这些变量能够正确地被提升，从而解决了在相互依赖的模块中过早访问导致的运行时错误。这一改变提升了 Turbopack 在复杂依赖场景下模块打包的健壮性。

Highlights

• 修复循环依赖问题: 解决了 Turbopack 在 ESM 导出代码生成中处理循环依赖时，由于使用 const 声明导致变量无法提升的问题。
• 更改代码生成逻辑: 将默认导出的变量声明从 const 更改为 var，以确保在存在循环依赖的情况下，变量能够正确提升。

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[gemini-code-assist]

Code Review

你好，感谢你的贡献。这个 Pull Request 通过将 ESM 导出代码生成中的 const 替换为 var，旨在解决循环依赖问题。这个改动是正确且有效的。通过使用 var，利用其变量提升的特性，可以避免在循环依赖场景中因“暂时性死区”（Temporal Dead Zone）而导致的 ReferenceError。这是一个处理此类问题的标准做法。我对 turbopack/crates/turbopack-ecmascript/src/references/esm/module_item.rs 和 turbopack/crates/turbopack-ecmascript/src/tree_shake/graph.rs 中的改动进行了审查，确认它们都符合预期，并且共同解决了所描述的问题。整体来看，这是一个高质量的修复，代码清晰，目标明确。我没有发现任何问题。