Fix install function

[MikeInnes]

After #190, install was broken due to some missing imports. Also, the behaviour of unpack seems to have changed: it now errors when used this way, because the target directory is required to be empty. I've added an unzip to a temporary directory in order to replicate the old behaviour.

I haven't checked for other uses of unpack, which may also need fixing, or for other missing imports; this just fixes one particular build script I have.