Add path to cache key for Rust dep inference, for relative imports (cherry-pick of #19630)

[huonw]

This adds the file path for the file being dependency-inferred to the cache key for the Rust dep inference. Without this, the same file contents appearing multiple times in different places will give the wrong results for relative imports, because the dep inference process mashes together the file path and the relative import.

The circumstances here seem most likely to occur in the real world if a file is moved, with the inference results from before the move reused for the file after the move too.

I've tested this manually on the reproducer in #19618 (comment), in addition to comparing the before (fails) and after (passes) behaviour of the new test.

To try to make this code more resilient to this sort of mistake, I've removed the top level PreparedInferenceRequest.path key, in favour of using the new input_file_path on the request protobuf struct, at the cost of an additional String -> PathBuf conversion when doing the inference.

Fixes #19618

This is a cherry pick of #19630, but is essentially a slightly weird rewrite that partially cherry-picks #19001 too, by copying over the whole DependencyInferenceRequest protobuf type as is (even with some fields that are unused) because that's the easiest way to generate appropriate bytes for turning into the digest for use in the cache-key. I think it's okay to have this "weird" behaviour in the 2.17 branch, with the real/normal code in main/2.18?