Only yield output neighbours in petgraph's IntoNeighbors trait

[aborgna-q]

No description provided.

[acl-cqc]

Yes, but it would be nice to trial this with Hugr first and see that it behaves as I expect; can you run me through how to do that?

[aborgna-q]

I checked it on hugr/main.

If you have a local clone, you can add a path = "../portgraph" attribute to the dependency in Cargo.toml.
Alternatively, you can get it from github

git="https://github.com/CQCL/hugr", branch="fix/petgraph-neighbours"

[acl-cqc]

Hmmmm, I put a panic in here and yet the Hugr code using petgraph DfsPostOrder doesn't panic. Maybe there's another place?

[acl-cqc]

Ok, so turns out this isn't the code we are using in Hugr, so we are at the level of "it looks wrong", but I'll stand by that it does look wrong and this PR looks like a fix and I have approved it :)

[aborgna-q]

It's part of the IntoNeighbors definition:

The neighbors are, depending on the graph’s edge type:

• Directed: All targets of edges from a.