This commit allows an mli-file to implement a module listed in the Modules
field of a library, provided that it only includes type and signature
definitions. The mli- and cmi-files will be installed along with the
Please note the change of the type of the lib_ocaml field. If this is considered part of a (semi-)public interface, we may want to be careful. If so, a suggestion is appreciated.
Support pure interface modules in libraries.