py: Add a "Pointed package" protocol

[aborgna-q]

Guppy can compile functions into hugr packages, which have a function identified as an entry point.
This is similar to tket2 circuits, which point to a region in a hugr.

We should unify this notion in a Protocol in hugr-py, so both libraries can interact without depending on each other.

class Proto(Protocol):
    def package(self) -> Package: ...
    def hugr_index(self) -> int: ...
    def root(self) -> Node: ...