Nickel LSP integration and packaging

[vkleen]

At the moment, rules_nickel support copying Nickel files into the a temporary directory before running nickel export as a method for handling dependencies. This is suboptimal from an LSP integration standpoint: the files cannot be found by the Nickel language server during development.

We need a way of integration with the language server to tell it about extra files. When Nickel gets an accepted way of packaging libraries, we need to integrate with that.

This issue probably needs upstream support from the Nickel project.