emsdk

If an archive has not been downloaded, download it and assume its contents are new (#248)

This allows us to always unpack our builds into upstream/ or fastcomp/, instead of creating a new directory each time. Since without this, we'd get told to download a new file, and the emsdk would see that where it would be unpacked had contents, and assumed that even though it wasn't downloaded it must be the same. So we'd silently skip it.

It's useful to always unpack into the same dir since it's easier for people that create their own .emscripten file (the location of emscripten etc. is all fixed under the emsdk), and also it avoids directories accumulating, which each take hundreds of megabytes, so over time it can get burdensome.