Rollup merge of rust-lang#131728 - jieyouxu:boopstrap, r=onur-ozkan

bootstrap: extract builder cargo to its own module

I was looking at our cargo rustflags/rustdocflags usages, and I found `builder.rs` to be a large
file which made it hard to digest. This PR tries to break out the cargo command wrapper parts to
its own submodule to make it easier to identify builder cargo-specific logic.

This PR:

- Extracts the cargo command wrapper to its own module and also move `Builder::{bare_,}cargo` impl
  to the submodule.
- Reorganizes some imports in `lib.rs` (no functional changes).
- Slightly adjusts some docs in `builder.rs`.

This PR is basically just moving code around, and should not contain any functional changes.

Before this PR, `builder.rs` was 2743 lines. After this PR, `builder.rs` is down to a more
manageable 1386 lines and `cargo.rs` is 1085 lines.

Author: matthiaskrgr