I do not recall exactly how the 0.18 download process works, but here’s what I would try.
If I remember correctly, the packages are all downloaded into folders in your project’s elm-stuff/ directory like this:
elm-stuff/0.18.0/Skinney/murmur3/2.0.6/
If I recall correctly, it downloads the package when it does not see a src/ directory on that path, so if you download the code yourself into that location, I think 0.18.0 will assume that it downloaded it in the past.
I’m not 100% certain that I am remembering all the details, but I think this is roughly what I would look into first!