Turbopack: really fix export collection (#80240)
- For `export default class Foo`, we don't insert a `var __TURBOPACK__default__export__ = Foo` variable because it should be a live binding. So put the correct name/syntax context into `ImportMap.exports`
- The `__TURBOPACK__default__export__` generated by module fragment tree shaking should use the same syntax context as the one generated by `module_item.rs` (which is `SyntaxContext(0)`).
