Ctype.lower_contravariant: avoid missing cmis when expanding types

[Octachron]

When expanding types in Ctype.lower_contravariant, it is currently possible to lose variance information due to a missing cmi. For example, if we have

(* original.ml *)
type 'a t = A
(* middle.ml *)
type 'a t = 'a Original.t = A
(* user.ml *)
let a: 'a Middle.t = let _ = ref 0 in A

compiling user.ml without original.cmi results in a error in 4.09 and trunk (but not in 4.08). Indeed, the typechecker ignore the reexport in Middle and, due to the missing cmi, infers that 'a Original.t is invariant in its first parameter.

This PR proposes to avoid this issue by expanding only type abbreviations in Ctype.lower_contravariant.

Note that this change of behaviour breaks the build of dune in the current 4.09 beta, so it probably needs to be backported to 4.09 .

[lpw25]

I believe that this code is correct.

People should be very careful with missing .cmi files -- the type-checker does its best but we make no guarantees that things will always work. Even so, this change in (unspecified) behaviour seems like an improvement.

[Octachron]

Cherry-picked on 4.09 as 4bc9f50