Store .kotlin_module files in MetadataSerializer
Compilation of top level functions/properties/typealiases results in a bunch of different .kotlin_metadata files, so we need to store names of these files to avoid scanning the file system in the compiler when loading code compiled by K2MetadataCompiler. For this, we reuse the PackageTable protobuf message, which is already used for exactly the same purpose in the JVM back-end
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