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`

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