Fix As_project.files_of_generators.

This would return the *non* generated files and would
pile a list of useless rm's in distclean.

For example in assemblage's Makefile:
```
distclean:: clean
        rm -f Makefile assemblage.install META doc/*.html
        rm -rf $(BUILDIR)/lib-assemblage/as_flags.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_flags.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_resolver.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_resolver.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_features.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_features.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_shell.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_shell.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_action.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_action.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_git.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_git.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_project.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_project.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_ocamlfind.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_ocamlfind.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_OCaml.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_OCaml.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_build_env.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_build_env.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_makefile.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_makefile.mli.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_opam.ml.ml
        rm -rf $(BUILDIR)/lib-assemblage/as_opam.mli.ml
```