Before this PR the coverage_tool automatically registered by rules_python was visible outside the toolchain repository. This fixes it to be consistent with non-bzlmod setups and ensures that the default coverage_tool is not visible outside the toolchain repos.

This means that the MODULE.bazel file can be cleaned-up at the expense of relaxing the coverage_tool attribute for the python_repository to be a simple string as the label would be evaluated within the context of rules_python which may not necessarily resolve correctly without the use_repo statement in our MODULE.bazel.