Move toposorting onto DiGraph directly, and cleanup error handling

[ItsDoot]

Objective

• Part of Extensible Schedule Graphs #20115
• Since Better ScheduleBuildError introspection and handling #20256, ScheduleGraph::topsort_graph no longer uses self, so let's put it directly on DiGraph.

Solution

• Moved ScheduleGraph::topsort_graph to DiGraph::toposort.
• Added DiGraphToposortError with Loop and Cycle variants, which is wrapped to replace ScheduleBuildError::HierarchyLoop, HierarchyCycle, DependencyLoop, and DependencyCycle.
• Added ScheduleBuildError::FlatDependencySort variant to detect issues specifically with the flattened dependency graph.
• Removed ReportCycles: DiGraph::toposort returns an error that should be wrapped at the callsite, negating the need for this type.
• Moved simple_cycles_in_component onto DiGraph (formerly a free-floating function).

Testing

Updated and reused current tests.

[hymm]

makes sense to me