fetch-pack: use new fsck API to printing dangling submodules

fetch-pack: use new fsck API to printing dangling submodules

Refactor the check added in 5476e1efde (fetch-pack: print and use
dangling .gitmodules, 2021-02-22) to make use of us now passing the
"msg_id" to the user defined "error_func". We can now compare against
the FSCK_MSG_GITMODULES_MISSING instead of parsing the generated
message.

Let's also replace register_found_gitmodules() with directly
manipulating the "gitmodules_found" member. A recent commit moved it
into "fsck_options" so we could do this here.

I'm sticking this callback in fsck.c. Perhaps in the future we'd like
to accumulate such callbacks into another file (maybe fsck-cb.c,
similar to parse-options-cb.c?), but while we've got just the one
let's just put it into fsck.c.

A better alternative in this case would be some library some more
obvious library shared by fetch-pack.c ad builtin/index-pack.c, but
there isn't such a thing.

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