Track unused Type Aliases, Module Types and submodules

[kit-ty-kate]

type t = string
module type U = sig val f : unit -> unit end
module type S = sig val f : int -> int end
module A : S
module B : S

neither t, U, S, A and B will not show up in the dead code analysis even if none of them are used.

I suspect anything type-related is probably harder to detect, but at the very least submodules should show up.

Moved from #16 (comment)

[fantazio]

Hi @kit-ty-kate
Thanks for filling out this issue.
I am planning work for the year (#46). Currently, your request is out of scope but is considered for the long term improvements. Chances are that this will be included in next year's plan.
I hope that's OK with you and remain open to any comment or argument to push this feature forward.