core: Move pack file parsing into core, add ostree_create_file_from_input() API

This moves us closer to consistently passing around a triple of:
  (GFileInfo *info, GVariant *xattrs, GInputStream *content)
Which will help the libarchive work.

src/libotutil/ot-gio-utils.h

@@ -29,6 +29,8 @@ G_BEGIN_DECLS
 
 #define OSTREE_GIO_FAST_QUERYINFO "standard::name,standard::type,standard::is-symlink,standard::symlink-target,standard::is-hidden,unix::*"
 
+GFileType ot_gfile_type_for_mode (guint32 mode);
+
 GFile *ot_gfile_new_for_path (const char *path);
 
 const char *ot_gfile_get_path_cached (GFile *file);