pathspec.h: move pathspec_needs_expanded_index() from reset.c to here

Method pathspec_needs_expanded_index() in reset.c from 4d1cfc1351
(reset: make --mixed sparse-aware, 2021-11-29) is reusable when we
need to verify if the index needs to be expanded when the command
is utilizing a pathspec rather than a literal path.

Move it to pathspec.h for reusability.

Add a few items to the function so it can better serve its purpose as
a standalone public function:

* Add a check in front so if the index is not sparse, return early since
  no expansion is needed.

* It now takes an arbitrary 'struct index_state' pointer instead of
  using `the_index` and `active_cache`.

* Add documentation to the function.

Helped-by: Victoria Dye <vdye@github.com>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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