This is a pre-req to deleting the `Place.memberPath` field. We no longer need
memberPath in the HIR now that we have PropertyStore and PropertyLoad. However,
scope dependencies use memberPath to track the precise fields that a computation
depends upon. This PR changes scopes dependencies to use a new
`ReactiveScopeDependency` type (Place + optional path), which allows the next PR
to remove `Place.memberPath`.