builtin/repack.c: rename many 'struct existing_packs' functions

builtin/repack.c: rename many 'struct existing_packs' functions

Rename many of the 'struct existing_packs'-related functions according
to the convention introduced in and described by 541204aabe
(Documentation: document naming schema for structs and their functions,
2024-07-30).

Note that some functions which operate over an individual entry in the
list of existing packs are prefixed with "existing_pack_" instead of the
plural form.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>