Update all submodules that rustbuild doesn't depend on lazily

This only updates the submodules the first time they're needed, instead
of unconditionally the first time you run x.py.

Ideally, this would move *all* submodules and not exclude some tools and
backtrace. Unfortunately, cargo requires all `Cargo.toml` files in the
whole workspace to be present to build any crate.

On my machine, this takes the time for an initial submodule clone (for
`x.py --help`) from 55.70 to 15.87 seconds.

This uses exactly the same logic as the LLVM update used, modulo some
minor cleanups:
- Use a local variable for `src.join(relative_path)`
- Remove unnecessary arrays for `book!` macro and make the macro simpler to use
- Add more comments

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