Concurrency safe `FileImporter`

We embed the jsonnet VM in our CI/CD infrastructure and instantiate a VM per generation cycle, but found that when using the same VM instance for all generations it's quite a bit faster (I expect because of caching as many of the generations use the same files). However, at the same time we found the `FileImporter` to be not concurrency safe. Would you be open to adding locking to the file importer?

As I was writing this it also occurred to me, is the jsonnet VM otherwise concurrency-safe? This particular issue we found quickly during development, but I'm curious if it would be more work than just making `FileImporter` concurrency safe.