This commit adds a new tool, `build-manifest`, which is used to generate a
distribution manifest of all produced artifacts. This tool is intended to
replace the `build-rust-manifest.py` script that's currently located on the
buildmaster. The intention is that we'll have a builder which periodically:
* Downloads all artifacts for a commit
* Runs `./x.py dist hash-and-sign`. This will generate `sha256` and `asc` files
as well as TOML manifests.
* Upload all generated hashes and manifests to the directory the artifacts came
from.
* Upload *all* artifacts (tarballs and hashes and manifests) to an archived
location.
* If necessary, upload all artifacts to the main location.
This script is intended to just be the second step here where orchestrating
uploads and such will all happen externally from the build system itself.
This involves hacking the code used to run cargo test on various
packages, because it reads Cargo.lock to determine which packages should
be tested. This change implements a blacklist, since that will catch new
crates when they are added in the future.
