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.

python/extractor/tsg-python/BUILD.bazel

@@ -1,5 +1,5 @@
-load("@py_deps//:defs.bzl", "aliases", "all_crate_deps")
 load("//misc/bazel:rust.bzl", "codeql_rust_binary")
+load("//misc/bazel/3rdparty/py_deps:defs.bzl", "aliases", "all_crate_deps")
 
 codeql_rust_binary(
     name = "tsg-python",
@@ -14,3 +14,8 @@ codeql_rust_binary(
         normal = True,
     ) + ["//python/extractor/tsg-python/tsp"],
 )
+
+exports_files([
+    "Cargo.toml",
+    "Cargo.lock",
+])