packfile: split out packfile list logic

In the next commit we're about to introduce the "packed" object database
source. This source will embed a packfile list, and consequently we'll
have to include "packfile.h" to make the struct definition available.
This will unfortunately lead to a cyclic dependency that we cannot
resolve with a forward declaration.

Split out the code that relates to the packfile list into a separate
compilation unit so that both "packfile.h" and "odb/source-packed.h" can
include it.

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

Makefile

@@ -1233,6 +1233,7 @@ LIB_OBJS += pack-refs.o
 LIB_OBJS += pack-revindex.o
 LIB_OBJS += pack-write.o
 LIB_OBJS += packfile.o
+LIB_OBJS += packfile-list.o
 LIB_OBJS += pager.o
 LIB_OBJS += parallel-checkout.o
 LIB_OBJS += parse.o