[fud2] update path finding to support multi state input and output

[jku20]

trafficstars

This PR begins progress towards addressing #1958. It updates the algorithm used by fud2 for finding paths from input files to output files to take in operations which have many inputs and outputs.

The behavior concurs with the current fud2 in some but not all cases and does not maintain fud2's always finding the shortest path of operations.

Specifying operations themselves and emitting the Ninja file still assume single state inputs and outputs. This will be changed in further PRs.