A tool to extract the dependencies (imports)

This is a feature request.

I use GNU Make and the `jsonnet` utility to generate a JSON from my `.jsonnet` files. It would generate a file suitable to consumed by GNU Make, similar to [`gcc -MD`](https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html). Note that it should do so only when output file is specified with `--output-file`/`-o` (otherwise it's complicated to decide the make rule target).

For example, if [the example mentioned on website](https://jsonnet.org/learning/tutorial.html#imports) is run with `-o output.json`, it would generate a file `output.d` with content

```make
output.json: imports.jsonnet martinis.libsonnet garnish.txt

martinis.libsonnet:

garnish.txt:
```