Package::from_hugr shouldn't wrap twice

[aborgna-q]

Given a dataflow-container hugr like

graph LR
    subgraph 0 ["(0) DFG"]
        direction LR
        1["(1) Input"]
        2["(2) Output"]
        3["(3) tket2.quantum.QAlloc"]
        3--"0:0<br>qubit"-->2
    end

Loading

Package::from_hugr wraps it into a module, but it adds an extra layer of indirection

 graph LR
    subgraph 0 ["(0) Module"]
        direction LR
        subgraph 1 ["(1) FuncDefn: #quot;main#quot;"]
            direction LR
            2["(2) Input"]
            3["(3) Output"]
            subgraph 4 ["(4) DFG"]
                direction LR
                5["(5) Input"]
                6["(6) Output"]
                7["(7) tket2.quantum.QAlloc"]
                7--"0:0<br>qubit"-->6
            end
            4--"0:0<br>qubit"-->3
        end
    end

Loading

Instead of nesting the DFG, we should make it a FuncDefn directly and avoid the extra layer of indirection.
This would facilitate roundtrips in tket2 :)