bootstrap: Handle dotted table keys when parsing bootstrap.toml Fixes rust-lang/rust#156948 `bootstrap.py` has a small hand-rolled reader that pulls the stage0 cargo and rustc out of `bootstrap.toml` before the real TOML parser is built. It only understood `[table]` headers, so the dotted form that `bootstrap.example.toml` now uses (`build.cargo = "..."`) was silently dropped, and the cargo/rustc lookup matched those keys in any table rather than only `[build]`. Teach the reader to strip an optional dotted prefix and treat it as the key's table, the same way `configure.py` does when it writes the file, and scope the cargo/rustc lookup to `[build]`. New tests cover the dotted form, equivalence with the section form, wrong-table rejection, and dotted target keys. *Disclosure: the code was written by AI (Claude); reviewed and tested by me.*