Support derive on renamed crate imports

[KodrAus]

One of the benefits of wrapping the custom derive in a const is that we can re-import the appropriate elastic_types/elastic crate and give it an alias even if the user has imported it under a different name.

This should be a simple matter of splitting the crate_root parameter into krate and path, so we can:

const _IMPL_BLAH: () = {
    extern crate $krate as _derive;

    impl _derive::$path::DocumentType for Blah { }
};