Provide an upgrade path to V0.10 while preserving existing directory layouts

The behavior of `pkg_tar`s `include_runfiles` attribute seems to have changed quite dramatically in the v0.10 release.

When combined with the `strip_prefix = "/"` attribute, the directory layout of the runfiles tree before it was just nested at the root of the tarball, i.e. a runfile at `$package_dir/path/to/file.yaml` would be found exactly in that place.

Since v0.10, the runfiles somehow end up at `$package_dir/$myservice.runfiles/$package_dir/path/to/file.yaml`.

This completely breaks our existing workflows and compatibility. The PR notes for this change (https://github.com/bazelbuild/rules_pkg/pull/754) in my opinion do not really capture the magnitude of this breaking change. Could you describe a way to preserve the old behavior? I think many users upgrading to v0.10 will run into this issue.