Rollup merge of #111955 - jyn514:step-refactors, r=clubby789

bootstrap: Various Step refactors

This ended up being a bunch of only somewhat related changes, sorry :sweat: on the bright side, they're all fairly small  and well-commented in the commit messages.

- Document `ShouldRun::crates`

- Switch Steps from crates to crate_or_deps where possible

    and document why the single remaining place can't switch

- Switch doc::{Std, Rustc} to `crate_or_deps`

    Previously they were using `all_krates` and various hacks to determine
    which crates to document. Switch them to `crate_or_deps` so `ShouldRun`
    tells them which crate to document instead of having to guess.

    This also makes a few other refactors:
    - Remove the now unused `all_krates`; new code should only use
      `crate_or_deps`.
    - Add tests for documenting Std
    - Remove the unnecessary `run_cargo_rustdoc_for` closure so that we only
      run cargo once
    - Give a more helpful error message when documenting a no_std target
    - Use `builder.msg` in the Steps instead of `builder.info`

- Extend `msg` and `description` to work with any subcommand

    Previously `description` only supported `Testing` and `Benchmarking`,
    and `msg` gave weird results for `doc` (it would say `Docing`).

- Add a `make_run_crates` function and use it Rustc and Std

    This fixes the panic from the previous commit.

- Allow checking individual crates

    This is useful for profiling metadata generation.

    This comes very close to removing all_krates, but doesn't quite -
    there's one last usage left in `doc`. This is fixed in a later commit.

- Give a more helpful error when calling `cargo_crates_in_set` for an alias

    Before:
    ```
    thread 'main' panicked at 'no entry found for key', builder.rs:110:30
    ```

    After:
    ```
    thread 'main' panicked at 'missing crate for path library', check.rs:89:26
    ```

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