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.