Properly handle multi-file modules in some places

On each change to the stdlib indexer, I noticed a bunch of modules randomly changing their paths, which I assume is because those modules had their definitions spread out across multiple files in a hash map, and the ordering of a hash map is non-deterministic so the results of an iteration kept changing. Since the indexer uses a single map module name -> path string, there will need to be a way to store that a module may have more than one file system path for its definitions, perhaps switching that path string to a list, making the necessary changes. I'm not sure if this is a problem outside of the stdlib indexer, but that's worth investigating as well.