fsck.c: move gitmodules_{found,done} into fsck_options

fsck.c: move gitmodules_{found,done} into fsck_options

Move the gitmodules_{found,done} static variables added in
159e7b080bf (fsck: detect gitmodules files, 2018-05-02) into the
fsck_options struct. It makes sense to keep all the context in the
same place.

This requires changing the recently added register_found_gitmodules()
function added in 5476e1efde (fetch-pack: print and use dangling
.gitmodules, 2021-02-22) to take fsck_options. That function will be
removed in a subsequent commit, but as it'll require the new
gitmodules_found attribute of "fsck_options" we need this intermediate
step first.

An earlier version of this patch removed the small amount of
duplication we now have between FSCK_OPTIONS_{DEFAULT,STRICT} with a
FSCK_OPTIONS_COMMON macro. I don't think such de-duplication is worth
it for this amount of copy/pasting.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>