odb/source-loose: wire up `reprepare()` callback

odb/source-loose: wire up `reprepare()` callback

Move `odb_source_loose_reprepare()` from "object-file.c" into
"odb/source-loose.c" and wire it up as the `reprepare()` callback of the
loose source.

While at it, make `odb_source_loose_clear_cache()` static, as it is no
longer needed outside of its file.

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