bazel: Qualify build_file attributes with @zephyr in MODULE.bazel

When Zephyr is consumed as an external dependency via Bzlmod, repository
rules defined in Zephyr's MODULE.bazel (such as new_git_repository for
cmsis and cmsis_6, and http_archive for toolchain SDKs) resolve bare
//... labels against the root repository rather than @zephyr.

Qualify these build_file paths with @zephyr// so that downstream
projects can consume them without repository resolution errors.

Change-Id: Ie1c3b92148336cbffbea7e677406a78a2bd7a84a
Reviewed-on: https://pigweed-review.googlesource.com/c/third_party/github/zephyrproject-rtos/zephyr/+/462405