depgraph: buidpkgonly error earlier, bug #471910
depgraph: tuple display list, avoid copies

TODO: Optimize _show_merge_list to use reference comparison more
instead of == comparison.
depgraph.display(): handle reverse for --tree

This factors --tree logic out of the calling code, and allows
optimization of _show_merge_list to use reference comparison instead
of == comparison. Also, deprecate the unused depgraph.altlist()
"reversed" parameter, due to builtin name collision.