A fix to the previous problem: it is enough to add a foo_bar.mllib

file, containing just the (packed) module Foo_bar. Then, compiling
foo_bar.cma with the command:

ocamlbuild -classic-display -Is src,src/bar src/bar/foo_bar.cma

ends with a correct library creation:

ocamlfind ocamlc -pack src/bar/a.cmo -o src/bar/foo_bar.cmo
ocamlfind ocamlc -a src/bar/foo_bar.cmo -o src/bar/foo_bar.cma

where module M does not appear. A general fix is then to create both
an mllib and an mlpack file when for packed libraries.

examples/issue-1239/src/bar/foo_bar.mllib

@@ -0,0 +1 @@
+Foo_bar