Fix NoneType error in rustdoc

[UebelAndre]

There seemed to be a confusion around CrateInfo.output being optional within the rustdoc rules that leads to a NoneType error when using rustdoc on a generated crate root:

external/rules_rust+/rust/private/rustc.bzl:1090:38: Traceback (most recent call last):
File "external/rules_rust+/rust/private/rustc.bzl", line 2883, column 16, in _get_dirname
    return file.dirname
Error: 'NoneType' value has no field or method 'dirname'

This change adds a proper output to satisfy the the current CrateInfo interface.

[UebelAndre]

@krasimirgg @slackito do either of you know if this would break anything on Google's end?

[krasimirgg]

I wonder if instead we could simulate this behavior by checking whether crate_info.output is a directory or a file? If I understand correctly, it's a directory if and only if we're in a rustdoc context, and it's exactly when we want to suppress emitting --out-dir?

[UebelAndre]

Done, now if output is a directory, we only emit --out-dir for that path but if it's a source we emit it for dirname.

[krasimirgg]

@krasimirgg @slackito do either of you know if this would break anything on Google's end?

we don't directly use the rustfmt aspect, but I'm not sure if we don't have other extensions that produce a CrateInfo with an empty output. Let me run a check about that...

EDIT: ran a sample test, that bit looks OK. Left a comment about potentially simplifying the logic by sniffing if crate_info.output is a directory or not.

[slackito]

Looks good to me. I'll report back if it breaks anything once we import it.

[slackito]

Actually I just raced with Krasimir and it turns out he had comments. Please address them before merging.