packfile: rename `struct packfile_store` to `odb_source_packed`

packfile: rename `struct packfile_store` to `odb_source_packed`

Not too long ago, we have introduced the packfile store in b7983adb51
(packfile: introduce a new `struct packfile_store`, 2025-09-23). This
struct is responsible for managing all of our access to packfiles and is
used as one of the two sources of objects for the "files" source.

Back when I introduced this structure I didn't have the clear vision yet
that it will eventually also turn into a proper object database source,
and how exactly that infrastructure will look like. Now though it's
becoming increasingly clear that it does make sense to treat it just the
same as any of our other ODB sources.

The consequence is that the naming is now a bit out-of-date: it's just
another source and will be turned into a proper `struct odb_source` over
the next couple of commits, but it's not named accordingly.

Rename the structure to `odb_source_packed` to align it with this goal
and to bring it in line with the other sources we already have.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>