Wheel examples fail to build when using fully qualified package path

@fweikert discovered that we fail to build in the Bazel federation's CI because they specify our `//experimental/examples/...` targets as `@rules_python//experimental/examples/...`, and this messes up the `py_package` filtering logic. The failure repros locally with e.g.

    bazel build @rules_python//experimental/examples/wheel:minimal_with_py_package