Do not include utility headers in `ranges_defs.h`

[mmichel11]

This issue was initially brought up in #2065 (comment).

ranges_defs.h adds our internal ranges along with nanoranges into our public namespaces. This header includes utils_ranges.h along with hetero/dpcpp/utils_ranges_sycl.h in order to obtain our internal range definitions. We include this header in ./hetero/dpcpp/parallel_backend_sycl.h and glue_algorithm_ranges_impl.h.

As suggested in the comment, we should not be include utility headers ranges_defs.h but rather provide forward declarations at this level to our internal range classes. This removes the risk of circular dependencies that was fixed in #2065.