Rollup merge of #91965 - ferrocene:pa-more-granular-exclude, r=Mark-Simulacrum

Add more granular `--exclude` in `x.py`

x.py has support for excluding some steps from the current invocation, but unfortunately that's not granular enough: some steps have the same name in different modules, and that prevents excluding only *some* of them.

As a practical example, let's say you need to run everything in `./x.py test` except for the standard library tests, as those tests require IPv6 and need to be executed on a separate machine. Before this commit, if you were to just run this:

    ./x.py test --exclude library/std

...the invocation would eventually fail, as that would not only exclude running the tests for the standard library (`library/std` in the `test` module), it would also exclude generating its documentation (`library/std` in the `doc` module), breaking linkchecker.

This commit adds support to the `--exclude` flag for prefixing paths with the name of the module their step is defined in, allowing the user to choose which module to exclude from:

    ./x.py test --exclude test::library/std

This maintains backward compatibility with existing invocations, while allowing more ganular exclusion. Examples of the behavior:

| `--exclude`         | Docs    | Tests   |
| ------------------- | ------- | ------- |
| `library/std`       | Skipped | Skipped |
| `doc::library/std`  | Skipped | Run     |
| `test::library/std` | Run     | Skipped |

Note that this PR only changes the `--exclude` flag, and not in other `x.py` arguments or flags yet.

In the implementation I tried to limit the impact this would have with rustbuild as a whole as much as possible. The module name is extracted from the step by parsing the result of `std::any::type_name()`: unfortunately that output can change at any point in time, but IMO it's better than having to annotate all the existing and future `Step` implementations with the module name. I added a test to ensure the parsing works as expected, so hopefully if anyone makes changes to the output of `std::any::type_name()` they'll also notice they have to update rustbuild.

r? `@Mark-Simulacrum`

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