Store explicit project on workspace member (#4048)
We know that `[project]` must exist for each workspace member, so we can
store it directly and avoid going through the `.and_then()` when we need
to access it. This requires cloning the struct due to lack of
self-referential structs. An alternative would taking the `Project` from
`PyProjectToml` instead, but this could be confusing when passing the
`PyProjectToml` around.
