Rust: Vendor 3rdparty dependencies.

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.

rust/codegen/BUILD.bazel

@@ -2,7 +2,7 @@ load("@bazel_skylib//rules:native_binary.bzl", "native_binary")
 
 _args = [
     "//rust/ast-generator",
-    "//rust/ast-generator:manifest",
+    "//rust/ast-generator:Cargo.toml",
     "//misc/codegen",
     "//rust:codegen-conf",
 ]