NoSimpleLetBody should report destructuring for named patterns in other modules where destructuring does not change the type

[jfmengels]

At the moment, the rule doesn't report the destructuring of single variant constructors when we don't know whether the type has any phantom types (as that could be the purpose of the destructuring).

This correctly doesn't get reported.

type A b = A String
a = let (A b) = xyz
    in A b

But if the outer function has a type signature, and the return type is the same as the type of the value, then there is no need to destructure the value.

module A exposing (..)
type A b = A String

-- should be simplified
xyz : A String
xyz = -- ...

-- Can be simplified
a : A String
a n = let (A b) = xyz
    in A b

This probably needs type inference to work correctly.