bazel: incompatible_config_setting_private_default_visibility

Visibility on config_setting isn’t historically enforced. This is purely for legacy reasons. There’s no philosophical reason to distinguish them.

This flag, in conjunction with --incompatible_enforce_config_setting_visibility (https://github.com/bazelbuild/bazel/issues/12932), removes that distinction.

Values:

• --incompatible_config_setting_private_default_visibility=off: if --incompatible_enforce_config_setting_visibility=off, every config_setting is visible to every target, regardless of visibility settings. Else, every config_setting without an explicit visibility setting is //visibility:public (ignoring package visibility defaults)
• --incompatible_config_setting_private_default_visibility=on: if --incompatible_enforce_config_setting_visibility=off, every config_setting is visible to every target, regardless of visibility settings. Else, config_setting follows the same visibility rules as all other targets.

Incompatibility error:

ERROR: myapp/BUILD:4:1: in config_setting rule //myapp:my_config: target 'myapp:my_config' is not visible from target '//some:other_target. Check the visibility declaration of the former target if you think the dependency is legitimate

Migration:

Treat all config_settings as if they follow standard visibility logic at https://docs.bazel.build/versions/master/visibility.html: have them set visibility explicitly if they’ll be used anywhere outside their own package. The ultimate goal of this migration is to fully enforce that expectation.