Support pure interface modules in libraries.

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
library.

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.