rustdoc: move `#[macro_use] extern crate` statements outside the generated main in doctests

[QuietMisdreavus]

In #48106 i moved plain extern crate some_crate statements outside the fn main that rustdoc generates, but i failed to take #[macro_use] attributes into account. Since one of the common reasons people would add the manual crate import is to import its macros, this makes the feature less useful.

(Mentoring instructions to follow...)

[QuietMisdreavus]

To make a PR for this, you can effectively follow the blueprint in #48106. There's a conditional in partition_source in src/librustdoc/test.rs that checks for inner attributes and extern crate statements, so the change is as small as adding another condition to check for lines starting with #[macro_use] extern crate as well. You should also add another test to the grouping of unit tests at the bottom of the file to make sure it works.

If you run into issues in the error index like i did in the original PR, we can deal with it after the PR is put up. If you want to take this PR on, comment here or find me in #rustdoc on IRC!

[memoryleak47]

I'd like to try this one. :)

[QuietMisdreavus]

Great! Let me know if you run into any issues or want some help. :D