Create pkg_expand to allow for combination of archives

`pkg_tar` has a unique ability to absorb other tarballs in its [`deps` attribute](https://github.com/bazelbuild/rules_pkg/blob/main/pkg/docs/reference.md#pkg_tar) into its output.

This should be generalized into allowing any archive to be expanded into another. As a start, this can simply support the extraction of tar and zip archives. Others can be added later if needed.