Properly deduplicate reexported default exports #2866
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Resolves #2865
Description
Default exports are tricky as they technically define new variables but can be simplified to existing variables in case an existing variable is default exported—as long as this variable is not reassigned. This logic already caused issues in the past and recently caused #2865 because past fixes did not take into account that variables could be default exported over several stages.
This PR fixes this by adding a general solution to find the best variable to represent a default export, avoiding cases where a chunk imports the same binding under different names and assigns them to the same name.
This will NOT remove duplicate exports of the same binding from a chunk. The reason is that in case the chunk is a static or dynamic entry point, we cannot just remove exports and in all other cases, some additional logic would be necessary to determine which exports are the unused ones.