core: INCOMPATIBLE CHANGE: Rename .packfile to .archive

It's too confusing that we call the mode "archive" but the actual
files ".packfile".  Also, git already has a "packfile" that serves a
totally different purpose.

src/libostree/ostree-repo.c

@@ -1919,7 +1919,7 @@ iter_object_dir (OstreeRepo             *self,
       if (g_str_has_suffix (name, ".meta"))
         objtype = OSTREE_OBJECT_TYPE_META;
       else if (g_str_has_suffix (name, ".file")
-               || g_str_has_suffix (name, ".packfile"))
+               || g_str_has_suffix (name, ".archive"))
         objtype = OSTREE_OBJECT_TYPE_FILE;
       else
         goto loop_out;