Update pre-commit for bazel mod deps (#3525)

I feel kind of weird adding a pre-commit for this, but I think it may
make validating #3524 easier.

scripts/bazel_mod_deps.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python3
+
+"""Run `bazel mod deps` for pre-commit. Lets pre-commit handle modifications."""
+
+__copyright__ = """
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+"""
+
+import subprocess
+
+import scripts_utils
+
+
+def main() -> None:
+    scripts_utils.chdir_repo_root()
+    bazel = scripts_utils.locate_bazel()
+    subprocess.run([bazel, "mod", "deps"])
+
+
+if __name__ == "__main__":
+    main()