We've been observing some performance issues using crate_universe on CI.
Therefore, we're moving to vendor the auto-generated BUILD files
in our repository. This should provide a nice speed boost, while
getting rid of the complexity of the "rust cache" job we've been using
when we had a lot of git dependencies.
This PR includes a vendor script, and I'll put up a CI job internally
that runs that vendor script on Cargo.toml and Cargo.lock changes, to check
that the vendored files are in sync.
This passes command line arguments to codegen, allowing in particular
`--force` to be passed.
Also, a convenience `//rust/codegen:py` is added to only run the python
based code generation, which will be faster and enough when
`ast-generator` is unchanged.
We've hardcoded the tilde in a lot of places :(
This improves performance on Windows and gets us ready for Bazel 8.
We need an upgrade of rules_rust for this to work.
