pack: move open_pack_index(), parse_pack_index()

alloc_packed_git() in packfile.c is duplicated from sha1_file.c. In a
subsequent commit, alloc_packed_git() will be removed from sha1_file.c.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-check.c

@@ -2,6 +2,7 @@
 #include "pack.h"
 #include "pack-revindex.h"
 #include "progress.h"
+#include "packfile.h"
 
 struct idx_entry {
 	off_t                offset;