Graph algorithms not yet compatible with multi-driven nets

[RenWal]

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Describe the bug The graph_algorithm plugin makes several references to net::get_source(), which is deprecated and superseded by net::get_sources(). This will break functionality if multi-driven nets are encountered in the netlist.

Expected behavior The plugin should be fully compatible with multi-driven nets.

(Documenting this here since I believe we never talked about this internally)