Compute the right output filenames for rustdoc invocations that request the JSON format

[LukeMathWalker]

What does this PR try to resolve?

cargo currently assumes that all invocations of cargo rustdoc produce HTML files as outputs.
This assumption breaks down for invocations of cargo rustdoc that instruct rustdoc to produce documentation in a different format (i.e. JSON).

As a consequence, running the very same command multiple times (cargo +nightly rustdoc --lib -p <NAME> -- -Zunstable-options -wjson) recomputes the documentation every single time, instead of detecting that the pre-existing file is still fresh.

This PR fixes the logic in cargo to correctly detect the output filename when the JSON format is required.

How should we test and review this PR?

I've added a test to confirm that:

• The output filename is what we expect (i.e. <crate name>.json);
• The unit is marked as fresh the second time the same command is invoked.

Additional information

An alternative implementation strategy could see us adding --output-format as a cargo rustdoc option, rather than trying to parse the extra arguments being passed to rustdoc.

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[ehuss]

Just FYI, cargo has a policy of not parsing rustc or rustdoc flags. Generally, if there is some behavior that cargo needs to be aware of, then that needs to be lifted up as a first-class feature where cargo is driving the process (such as with its own flags). I don't offhand know how this should be designed, but I think it would be good to at least start with an issue where that design could be discussed.

[LukeMathWalker]

I see, then I'll pursue the alternative I listed in the PR description—lifitng --output-format to a flag of cargo rustdoc.
I'll open an issue about it 👍🏻

[LukeMathWalker]

Opened #12103