Rewrite symlinks for vendored repositories

To make sure symlinks work correctly, Bazel uses the following strategy to rewrite symlinks in the vendored source:

  - Create a symlink `<vendor_dir>/bazel-external` that points to `$(bazel info output_base)/external`. It is refreshed by every Bazel command automatically.
  - For the vendored source, rewrite all symlinks that originally point to a path under `$(bazel info output_base)/external` to a relative path under `<vendor_dir>/bazel-external`.

Fixes https://github.com/bazelbuild/bazel/issues/22303

Closes #22723.

PiperOrigin-RevId: 644349481
Change-Id: I853ac0ea5405f0cf58431e988d727e690cbbb013