Fix for unintentional filename conflicts within raw_models in streaming

[rokuniichi]

Solution for this issue:
#119

[thelink2012]

Thanks @rokuniichi and sorry for the late review.

I have replaced the std::map<filepath, ...> because during the non-streaming search it'd do O(n) lookups with std::find_if. As a replacement solution, I am moving off std::map<filename, ...> to std::multimap<filename, ...>.