Add additional modules to module_ctx about bazel

[Wyverald]

This sounds good to me (except maybe if we can find a better name for current_module).

The implementation is quite simple (file in question: https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleExtensionContext.java), so contributions are welcome.

[fmeum]

In the module extensions I have worked on so far, the usual pattern has been:

def _impl(module_ctx):
  collected_data = ...

  for mod in module_ctx.module:
      if mod.is_root:
          do_stuff(collected_data)

      do_other_stuff(collected_data)

I can see the new helpers being useful in smaller module extensions, but I think that many (most?) extensions will want to do something (even if just validation) with all modules, not just root and "current" module. The new symbols would get us into a situation where do_something(module_ctx.root_module), if mod.is_root: do_something(mod) and if mod == module_ctx.root_module: do_something(mod) are all different ways to do the same thing.

I am a bit worried that introducing new API that doesn't provide any truly new information could end up making module_ctx more confusing.