Turbopack: allow `get_definable_name` to return a list (#97984)
Followup to #95310
Now we can correctly track env vars in cases like these were a
`JsValue::Alternative` is involved
```js
// An alternative (process|unknown).env
let p2
if (foo) {
p2 = foo
} else {
p2 = process
}
console.log(p2.env.FOO8)
```
The inlining logic behaves the same as before:
- If `get_definable_name` returns exactly one result, we can inline.
- Otherwise, we can't inline (previously this case returned `None`)
A bit of trivia why we need that boolean in the return type in the first
place (because conceptually it should be possible to just model it via
`JsValue::Alternative` anyway:
> but why is that a problem? if the value is reassigned, then there
should just an alternative, then get_definable_name returns multiple
values and it should still not be inlining
>
> Because module is a `FreeVar`, not a normal `Variable`.
>
> The assignment is stored separately in `VarGraph`:
>
> ```
> free_var_ids["module"] -> id
> values[id] -> assigned function
> ```
>
> But later reads still evaluate to:
>
> `JsValue::FreeVar("module")`
>
> The linker expands JsValue::Variable, but not JsValue::FreeVar.
Therefore there is no JsValue::Alternatives at this call site.
>
> Instead, `get_definable_name()` detects the graph entry and returns:
>
> `[Some((["module", TypeOf], true))]`
>
> That boolean is how reassignment is represented for free variables.
Cardinality remains one, so ignoring the boolean at
references/mod.rs:4007 incorrectly permits inlining.
>
> Actual `Alternatives` primarily represent multiple values assigned to
tracked variables. For free globals, the original ambient value plus
reassignment is represented by `FreeVar` plus `potentially_reassigned`.
No perf impact:
```
commit cfc7da3049f91669a1b61a0f7a5edef503cddc53 (HEAD -> canary, origin/canary, origin/HEAD)
393.17s user 31.34s system 722% cpu 58.775 total
391.91s user 35.37s system 746% cpu 57.256 total
389.48s user 30.40s system 770% cpu 54.513 total
commit 38f2708659be31538c951b12ba806ef53710f074 (HEAD -> mischnic/get-definable-name-list)
392.86s user 34.00s system 745% cpu 57.221 total
388.10s user 32.72s system 766% cpu 54.912 total
391.84s user 37.82s system 717% cpu 59.865 total
```
