feat(typing): support approximating modular explicits in let rec

[johnyob]

Context

This PR builds upon #14394, #14538 to support approximating module dependent function types when typing let rec.

Description

This PR extends type_approx to handle module dependent functions fun (module M : S) -> e.

The approximation is

type_approx env (fun (module M : S) -> e) = 
  with_moddep_param env M S (fun env ->
    type_approx env e) 

The cheeky helper with_moddep_param does the following:

1. Enter a new scope with a freshly created scoped ident that is used to bind the module type from the package type S
2. Call the function passed to it with the extended environment
3. Uses Ctype.instance_funct_opt to replace the scoped ident with an unscoped one
4. Profit :)

Reviewing

Warning

#14394, #14538 should be reviewed first -- this PR builds directly on top of it

This PR is best reviewed commit-by-commit:

1. The first commit extracts with_moddep_param from type_moddep_fun
2. And the second implements the approximation of module dependent types in Type_approx

Testing

make -C testsuite one TEST=tests/typing-modular-explicits/general.ml