* Enforce layering_check in Bazel build rules.
Enforcing layering_check ensures that the Build targets do not rely on
transitive dependencies. See
https://github.com/bazelbuild/bazel/pull/11440 for a detailed
description of the feature.
We also do a style pass on the build files, ensuring that common linters
are happy with it.
* Add .bazelversion file and fix build_defs.bzl.
We fix build_defs.bzl to work on Bazel 5.0.0.
