Exract manifest handling into its own crate

The build-manifest tool inside rust-lang/rust is responsible for creating manifests, and the manifest module in rustup-dist is responsible for reading and validating manifests. There have already been bugs where manifests produced by the Rust build system failed to validate. It would be preferable if the code was in a single crate and shared between the two projects.

Extract manifest.rs into its own crate, then plug that crate back into rustup, and into build-manifest. Modify build-manifest to run the validate method before writing the manifest. Extract manifest-specific tests as well. Look for other code to share between them.

Start with the manifest module in rustup-dist and the build-manifest tool in rust-lang/rust, then inspect the manifest-specific tests and the existing validate method. Extract the shared manifest handling into a crate, integrate it with both consumers, and verify that build-manifest validates manifests before writing them.