Add `serial-buildorder` argument to the build-tree

[ammernico]

• Introduced a new command-line argument --serial-buildorder that outputs the tree-of package in a serial build order.
• The --serial-buildorder argument conflicts with the existing --dot argument for now. Showing the serial build order in the dot format would not bring any benefits.
• Ensured cyclic dependency detection and error handling during topological sorting.

[christophprokop]

tested successfully :) thx!