What type of PR is this?
Feature

What does this PR do? Why is it needed?
After #4268, nogo collects facts from all targets. So go_register_nogo's includes and excludes now overlap with the JSON config's only_files and exclude_files, which is to surppress nogo errors in certain files/targets. Meanwhile, collecting facts can be slow for very large Go packages for some analyzers, slower than compilation. To address the performance issue, this PR revert excludes to its previous behavior: stop nogo from collecting facts from certain packages. To avoid collecting facts from IDL-generated code, users can do this:

    go_register_nogo(
        nogo = "@//:default_nogo",
        excludes = ["@//idl:__subpackages__"],
    )

The includes of go_register_nogo a no-op with a warning message, so nogo collects facts from all packages unless excluded explicitly. Users have to use the JSON config to limit the scope of nogo validation.

This PR also exclude all external targets from nogo validation by default, which is equivalent to

{
  "_base": {
    "exclude_files": {
      "^external/": "external repositories"
    }
  }
}

Other notes for review
Alternatively, to be backward-compatible, we can add a flag or attribute like exclude_collect_facts, but adding that on top of excludes and exclude_files makes hard to remember which is which.