* Merge multiple symbols even when re-exported
As far as I remember, the target of `mergeSymbol` needs to be a merged
symbol, not a symbol with a mergeId that points to mergedSymbol.
However, mergeSymbolTable didn't check for this.
I can't remember if symbol tables may contain
symbols-with-mergeId. If they can, then mergeSymbolTable needs to call
getMergedSymbol on the individual targets of the merge. That's what I
did in this PR.
* Call getMergeSymbol eagerly
On the source, not target, of mergeSymbolTable's contents
dd98c17b10