Migrate to Bzlmod for managing external dependencies

[meteorcloudy]

With future Bazel LTS versions, Bzlmod will become the default system for external dependencies management.

Please consider migrating all your external dependencies in WORKSPACE to MODULE.bazel

For more information, please check bazelbuild/bazel#18958

[meteorcloudy]

Some tests are still failing with Bzlmod enabled:
https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/3352#018af628-22bd-468d-8813-0356ebe78c86

[rickeylev]

posterity: From a cursory look, I get the impression the cause is some tests aren't going through the runfiles apis, so end up not being aware of repo-remapping.

[rickeylev]

PR #1482 out to fix this. It's as I thought -- the runfiles directory changing to _main broke various assumptions.