Preserve syntax of module unpack with type constraint ('((module X) : (module Y))' vs '(module X : Y)')

[gpetiot]

Extracted from ocamlformat-ng's concrete AST, replacing the convoluted pattern Ppat_constraint(Ppat_unpack x, Ptyp_package y) with Ppat_unpack(x, Some y)

Preserving this syntax (and not removing/adding keywords) makes the formatting of comments easier.