findAllReferences: In export default foo, symbol name is foo

[ghost]

Fixes #15814

Also, should be OK to return undefined from symbolName, since the result is just compared to the current search name to determine whether we should recursively search.

[DanielRosenwasser]

Does this also cover cases like the following?

export default Foo.Bar;

or

export default (Foo);

[ghost]

The purpose of this function is that if you have import X, we will recursively search through the original export if it is export default X but not if it is export default Y. We can't always infer a name for every expression since you could write export default 1 + 1;.
For a property access export default x.y, find-all-refs on y won't currently get references to imports of that default export even if I update symbolName, because the property symbol and exported symbol are different things. You could make an issue if in general you want references to a property access to include references to imports of exports that access that property.
I could add special handling for the case (foo) but I don't think that would really be worth it, since that doesn't seem likely to happen.