Only track pending imports for real promises (#95789)
Extracted from https://github.com/vercel/next.js/pull/95468.
Fixes a module with `then` export hanging the module tracking during the
build.
---
`trackPendingImport` needs to track actually async modules (top-level
await).
Before this change, this codepath was happy to adopt any thenable. But
since thenables may not be Promises, this could break things downstream,
hence why we had `Promise.resolve` there.
However, we don't *want* to adopt arbitrary thenables here. **Tracking
is for top-level awaited modules.** For example, `module.exports = new
Promise(() => {})` has a `.then` but we don't want to track that. It's
not TLA.
This is why, to catch real top-level await modules, we narrow the check
to `instanceof Promise`.
We add tests for:
- A client module with a `then` function export (**hangs** on Turbopack
before this PR with CC on)
- Exporting a literal Promise (works before and after, but worth testing
for future regressions)
