Fix SSR externalize inconsistencies

[bluwy]

Problem

Externalized dependencies in Vite has an inconsistency, where:

• In dev, Vite proxies importing the module (e.g. __esModule handling)
• In prod, Vite doesn't. Simply left the import as is for nodejs to load

Possible solutions

1. Make prod match dev behaviour (proxy everywhere)

This means wrapping all externalized modules with the proxy. However, I find this hard to handle. For example,

import { named } from 'some-cjs-lib-that-cant-be-named-imported'
// ^ this works in dev, but not in prod. in prod we have to rewrite this
// statement as `import * as pkg ...` and `pkg.named`

There's also the risk of some modules we're unable to detect to conditionally apply the interop. Plus existing issues of pkg.named auto-interop strategy that breaks live binding.

It's not impossible to support this, but takes a lot of work to do so.

2. Make dev match prod behaviour (no proxy)

This means no proxying at all. Dev should simply import the module. Feels like the right thing to do, and the fact that it would fail in prod anyways, maybe it doesn't be breaking much?

Feedback

I'd like feedback on:

1. Putting aside the work it'll take, which options to you prefer?
2. Concerns on the proposed solutions.
3. Other ideas.

[sapphi-red]

I considered the consistency with the type definitions and thought 2 is better.
If the package expects to be transpiled, then the package should be no-externalized. If the package expects to be also used without transpilation, then the package should ship a type definition compatible with Node's behavior. So in both cases, Ithink the type definition would work.

[bluwy]

I have made a PR for solution 2: #14521. Solution 1 is still on the table, but there hasn't been a discussion for it yet.

[bluwy]

Solution 2 is merged.