Support for squashing multiple input directories in pkg_files

If you have custom rules that generates a directory with `ctx.actions.declare_directory` you cannot squash them using pkg_files. By squashing I mean removing top directory specified by `ctx.actions.declare_directory`. This works with pkg_tar but not with the new pkg_files.

`strip_prefix` gives error right away saying it is not supported for TreeArtifacts. `renames` does not support multiple destination paths that are the same. This makes sense sometimes when you want to guarantee nothing is overwritten but it would be great to have an option to allow this behavior.