Fix attributes being dropped from packed module expressions and patterns

[tdelvecchio-jsc]

In the following program, attributes [@attr2] and [@attr3] are both dropped by ocamlformat silently:

module _ : sig
  val foo : (module T with type t = 'a [@annot1]) -> unit
end = struct
  let foo (type a) (module M : T with type t = a [@annot2]) =
    (module M : T with type t = a [@annot3])
  ;;
end

In the standard parser, expressions and patterns like (module M : S) are parsed as constraints wrapping an inner expression / pattern, while in the extended parser, the package type S is part of the same node as the module expression / pattern. In the parsing logic for this, the attributes parsed for the package type are just dropped.

In the second commit, we add the attributes to the package type, and handle it as necessary in all code.

[dvulakh]

Looks good to me. We agreed it would be better to wrap the package type in parentheses for clarity:

(module M : (T with type t = a)[@attr])

Instead of

(module M : T with type t = a[@attr])

But we decided not to block the bugfix on this and have filed a ticket internally instead.