MultiInfo is no longer preserved through serialization + parsing

[ekiwi]

It looks like since #2212, when we serialize a MultiInfo it can be turned into a single FileInfo node. This is a problem for applications that need to parse the emitted source locator, like my formal verification library: https://github.com/ekiwi/kiwi-formal/blob/main/test/kiwi/formal/backends/ErrorParserTest.scala#L39

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[ekiwi]

This is going to be fixed in: #2265