Just write yourself a file p.mli without your private modules:
=== p.mli ===
module Foo : ...
module Bar : ...
==========
Of course, you can not refer to signatures of modules Foo and Bar in p.mli. Thus you have to duplicate them or, better, to write them somewhere outside the package.
From a compilation point of view, be sure to generate p.cmi by using p.mli before generating the pack files
p.cm[ox].
We use such a trick in the Frama-C tool (
http://frama-c.com) for providing (usually empty) interfaces of its plug-ins. But I do not recommend you to read the Frama-C Makefiles for looking at the corresponding compilation lines (except if you are **very** motivated).
Hope this helps,
Julien