submodule: convert get_next_submodule to not rely on the_index

submodule: convert get_next_submodule to not rely on the_index

Instead of implicitly relying on the global 'the_index', convert
'get_next_submodule()' to use the index of the repository stored in the
callback data 'struct submodule_parallel_fetch'.

Since this removes the last user of the index compatibility macros,
define 'NO_THE_INDEX_COMPATIBILITY_MACROS' to prevent future users of
these macros in submodule.c.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>