Don't touch disk when handling zipped imports

[aednichols]

In #4406, we added a cleanup routine that deletes unzipped imports when we're done with them.

It would be nice to handle imports without touching the disk at all - see this branch for a mostly-working implementation

That implementation is 99% complete but seems to suffer from a Scala bug in certain packaging/deployment configurations - such as the one used in Travis!

[aednichols]

Because eliminating zip imports entirely is another "some day" goal, there is a world where we never even want to do this.