Support transformations with external modules

[imaustink]

When running transformations that use external modules, bee should download said modules before running, and copy the transformation into an internal folder before executing.

This folder should be named with a hash of the sorted modules names, and the transformation name, concatenated.

If the folder name already exists, there is no need to reinstall the modules but the transformation should be copied over again to prevent the potential for overlap.

With this setup, I'm going to need a way to clean up old folders. I am thinking read the dir, filter out non-dirs, and when the length of the result is over X, delete the oldest dir.