core: [INCOMPATIBLE CHANGE] Re-split archive content into .file and .filecontent

This will allow us to use hard links again for user-mode checkouts,
rather than the hackish link cache.  It was pretty silly anyways to
have file objects be stored with just a small metadata header
prepended, but uncompressed.

Either they should be hardlinkable, or compressed (in pack files).

src/libostree/ostree-core.h

@@ -169,6 +169,8 @@ void ostree_object_from_string (const char *str,
 char *ostree_get_relative_object_path (const char        *checksum,
                                        OstreeObjectType   type);
 
+char *ostree_get_relative_archive_content_path (const char        *checksum);
+
 char *ostree_get_pack_index_name (gboolean        is_meta,
                                   const char     *checksum);
 char *ostree_get_pack_data_name (gboolean        is_meta,