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-repo.h

@@ -77,6 +77,9 @@ GFile *       ostree_repo_get_object_path (OstreeRepo   *self,
                                            const char   *object,
                                            OstreeObjectType type);
 
+GFile *       ostree_repo_get_archive_content_path (OstreeRepo    *self,
+                                                    const char    *checksum);
+
 GFile *       ostree_repo_get_file_object_path (OstreeRepo   *self,
                                                 const char   *object);