Modularize rules_nixpkgs

[aherrmann]

Is your feature request related to a problem? Please describe.

Bazel 5.0 introduces a new dependency management system called "bzlmod", introduced in this talk. In that system, Bazel modules need to define metadata about themselves and their dependencies in a MODULE.bazel file and be registered in the Bazel Central Registry.

With Bazel 6, bzlmod became generally available (needs to be enabled with --enable_bzlmod). In Bazel version 5.0 it is introduced as an experimental feature.

As of now, rules_nixpkgs is not compatible with this new module system.

Describe the solution you'd like

• rules_nixpkgs should first be split into separate components for the separate language integrations.
  See Split rules_nixpkgs into separate components #182 for rational and details.
• Each component should then be given a MODULE.bazel file.
  Refer to "Modularize" rules_sh rules_sh#18 for an example.
• Split out testing modules and test bzlmod mode on CI. #322
• The existing repository rules offered by rules_nixpkgs need to be made compatible with and exposed as module extensions.
  See Create module extensions #183 for rational and details.
• update the examples or add examples to use bzlmod.
  Use a local registry or local_path_override to depend on the modules of rules_nixpkgs and ensure the example builds on CI (for each toolchain)
• The new release process should be documented.
  It should capture the multiple components and the BCR registration process (see Publish to BCR).
  Ideally, also provide some tooling to automate the process. E.g. as done in the bazel-contrib/rules-template.
• The components should be registered in the Bazel Central Registry.
  Refer to Add rules_sh 0.2.0 bazelbuild/bazel-central-registry#38 as an example.
• Generate, update, and test the generated documentation in bzlmod mode.
  This depends on Bzlmod: stardoc is broken by repo mappings bazelbuild/bazel#14140

Tasks

Beta Give feedback

1. Split rules_nixpkgs into separate components #182
   P2 type: feature request +2
2. Introduce a data attribute to nixpkgs_package #18
   +0
3. Split out testing modules and test bzlmod mode on CI. #322
   P2 +1
4. Create module extensions #183
   1 of 2
   +0
   
5. Update the examples or add examples to use bzlmod. #426
   +0
6. Document the new release process under bzlmod. #427
   +0
7. Register all rules_nixpkgs components using the [BCR app](https://github.com/bazel-contrib/publish-to-bcr). #428
   +0
8. Generate and update documentation in bzlmod mode. #429
   +0

Additional context

The rule author SIG has a discussion about bzlmod here.
The following blog posts may be useful resources for this work:

• https://blog.aspect.dev/bzlmod
• https://blog.rocketpoweredjetpants.com/2022/01/bzlmod-modules/
• bzmlod migration guide