The problem with your packages.tgz example is that you use "module
type Foo = .." in the .mli. This gives the signature of a module type,
that is, it refers to a _module type_ defined in the implementation
file.

What you want to do here is to give the signature of a _module_, not a
module types, so here is the correct syntax :

  module Foo : sig
    type foo_t

    val initial : foo_t
    val show : foo_t -> string
  end


Regarding your original problem, I've had the same needs and came up
with a slightly different solution : in order to avoid the additional
indirection level related to -pack (Foobar.Foo), is used a flattened
representation by adding a "foobar.ml" file containing only :

  include Foo

(and possibly include of other modules in the package). Then the
foobarl.mli is :

  type foo_t

  val initial : foo_t
  val show : foo_t -> string

And values can be referred with Foobar.foo, instead of Foobar.Foo.foo.
Of course this is only useful if you don't want the user to see the
internal module hierarchy, wich may not be what you had in mind.