object-file: move `force_object_loose()`

object-file: move `force_object_loose()`

In the preceding commits we have refactored `force_object_loose()` to
not call internal functions anymore for writing the object. Instead, it
now only uses generic functions that are accessible to all callers.

Consequently, we can now easily move the function to its only caller,
which is git-pack-objects(1). Do so.

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