[bazel] Make labels to third-party dependencies explicit

Prefix occurrences of `//utils/bazel` with an explicit `@llvm-raw`.

This change lets us reuse code from `configure.bzl` in future compatibility patches for the bzlmod
module system.

The llvm-project overlay will be made available as an `@llvm-project-overlay` (name WIP) module in
the Bazel Central Registry. This means that we will have an `@llvm-project-overlay` workspace in
addition to the `@llvm-raw` and `@llvm-project` workspaces currently involved in the build. To keep
future patches to the existing build files as small as possible, the explicit naming proposed in this
change appears to be the simplest way to not confuse the module workspace resolution.

This is not a functional change to the current WORKSPACE build. It is a foundation for future patches.

GitHub Issue in the BCR: https://github.com/bazelbuild/bazel-central-registry/issues/206
GitHub Issue in LLVM: https://github.com/llvm/llvm-project/issues/55924

Reviewed By: csigg

Differential Revision: https://reviews.llvm.org/D136496