In the native rules we have the late-bound attribute :lcov_merger which looks at the build configuration to see if coverage is enabled and returns the label of CoverageOutputGenerator or null (see BaseRuleClasses#115).

We need the Starlark rules to be able to define _lcov_merger as a late bound attribute in the same way. For this to happen, the CoverageOutputGenerators label has to be exposed on a class that extends BuildConfiguration.Fragment.

This is helpful for example for rules_go (see #8670), in order for go_test to be compatible with bazel's native test rules (e.g. java_test, cc_test).