This is a fairly well known problem. Hopefully it will be fixed in a few days when Pkg3 is released, but in the meantime, this has helped me:
https://github.com/KristofferC/Pkg25.jl
I think that the problem is that, when the package system is trying to work out how to satisfy the dependencies of a new package, it must read many small files in the Metadata repo. Normally this is fine, but on clusters (or other systems with remote file storage), it takes a relatively long time to access a file, and so Pkg appears to hang.