Step towards #5027 -- scopes should know if they are dynamic

The logic for distinguishing dynamic / static scopes was borrowed from Nick &
Paul's patch, the main differences here being 1) calling it "dynamic" (true/
false) rather than "parent_relationship" (:inherited/:dynamic) 2) aligning the
default so that it only needs to get set in one place (the one that will
eventually go away) and 3) setting it on createion rather than with a setter.

Setting it in one place, on creation, also makes it easier to see that anytime
we access a scope it will have the correct setting of Scope#dynamic and that
this does not change.

This commit also contains a minor refactor (removing Type#subscope) that is not
strictly tied to the main purpose but lies in the direction we are needing to
go and it simplified things to do it now.