pkg_tar for Python puts runfiles in wrong location

When I construct a pkg_tar rule to pull in a py_binary, e.g.

```
  pkg_tar (
      name = "pkg_name",
      srcs = [
          "//my/python/foo:bar",
      ],
       include_runfiles = True,
  )
```

Include_runfiles just dumps all files into the root directory, and doesn't create .runfiles directory for the foo:bar target, so that running bar fails because no runfiles are found. 