Examples for bazelbuild/proposals#269

1. Build and use Bazel from https://github.com/fmeum/bazel/tree/bzlmod-runfiles for the following steps.
2. In the root of this repository, run bazel run //:java_tool and bazel run //:some_tool to see examples of repository mapping manifests for Java and C++ targets.
3. In the other_module subdirectory, run bazel run @my_module//:java_tool and bazel run @my_module//:some_tool to see the same examples from the context of a different root module importing my_module.