This patch adds support for `[patch]` sections in `.cargo/config.toml`
files. Patches from config files defer to `[patch]` in `Cargo.toml` if
both provide a patch for the same crate.

The current implementation merge config patches into the workspace
manifest patches. It's unclear if that's the right long-term plan, or
whether these patches should be stored separately (though likely still
in the manifest). Regardless, they _should_ likely continue to be
parsed when the manifest is parsed so that errors and such occur in the
same place regardless of where a patch is specified.

Fixes rust-lang#5539.