odb: move initialization bit into `struct packfile_store`

odb: move initialization bit into `struct packfile_store`

The object database knows to skip re-initializing the list of packfiles
in case it's already been initialized. Whether or not that is the case
is tracked via a separate `initialized` bit that is stored in the object
database. With the introduction of the `struct packfile_store` we have a
better place to host this bit though.

Move it accordingly. While at it, convert the field into a boolean now
that we're allowed to use them in our code base.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>