build: Migrate to Bazel Module

This migrates from the WORKSPACE system to using the Bzlmod system.

These dependencies are now fetched with bazel_deps:

• googletest: updated to 1.16.0 (latest)
• eigen kept at 3.4.0 (latest stable)

Other dependencies are not in the Bazel Central Registry, so they are loaded as non-module extensions. Note that bazel_skylib is automatically resolved, and does not need to be included.