MPR#5185: creates incomplete cma under some circumstances

PR transferred from https://caml.inria.fr/mantis/view.php?id=5185
[original reporter: @ygrek]

Consider the following mllib:

X Q Y

Create in the current directory empty files x.ml and y.ml AND q.mli
Under normal circumstances ocamlbuild will notice missing implementation of Q module and build fails.
But if the name of the mllib file matches the name of one of the modules inside it - build succeeds with incomplete cma - e.g. with y.mllib we get:

$ PATH=/opt/ocaml-3.12.0/bin:$PATH ocamlbuild -classic-display y.cma
/opt/ocaml-3.12.0/bin/ocamldep.opt -modules x.ml > x.ml.depends
/opt/ocaml-3.12.0/bin/ocamldep.opt -modules q.mli > q.mli.depends
/opt/ocaml-3.12.0/bin/ocamlc.opt -c -o q.cmi q.mli
/opt/ocaml-3.12.0/bin/ocamldep.opt -modules y.ml > y.ml.depends
/opt/ocaml-3.12.0/bin/ocamlc.opt -c -o x.cmo x.ml
/opt/ocaml-3.12.0/bin/ocamlc.opt -c -o y.cmo y.ml
/opt/ocaml-3.12.0/bin/ocamlc.opt -a y.cmo -o y.cma

Note: see the Mantis discussion for more information and patches.

Reproduce the case with x.ml, y.ml, q.mli, and y.mllib using the ocamlbuild command shown in the report, then inspect the mllib handling involved in producing y.cma. Done means the missing Q implementation is rejected instead of producing an incomplete archive, while the normal X Q Y case still builds correctly.