Binary dependencies

[Keno]

@barche

I think for the official Julia binaries, BinaryBuilder is great here. The question is if the prefix should be inside the package directory, or if it should be a single prefix per environment for example? The advantage of a single prefix is that it simplifies configuration of binary packages that depend on each other, and on Windows this dir could be added to the path so the dlls of dependencies do not have to be loaded manually by the package. The downside is that there might be conflicts…

The rough plan is to not have a prefix at all (and have the julia dynamic library loader resolve the dependencies appropriately) or have a single prefix per environment that installs the correct versions of things, in order to make sure that a MANIFEST is always reproducible.