Demonstrate re-exports bug (#12783)

* test: demonstrate issue with unused-libs and re_exports

Signed-off-by: Rudi Grinberg <[email protected]>

Author: rgrinberg

test/blackbox-tests/test-cases/unused-libs/unused-libs-re-exports.t

@@ -0,0 +1,40 @@
+Demonstrate that re_exports is incorrectly triggering the unused library check:
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.21)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name foo)
+  >  (modules foo))
+  > (library
+  >  (name bar)
+  >  (modules bar)
+  >  (libraries (re_export foo)))
+  > EOF
+
+  $ touch foo.ml
+  $ cat >bar.ml <<EOF
+  > let x = ()
+  > EOF
+
+  $ mkdir use
+  $ cat >use/dune <<EOF
+  > (executable
+  >  (libraries bar)
+  >  (name use))
+  > EOF
+
+  $ cat >use/use.ml <<EOF
+  > let () = Bar.x
+  > EOF
+
+  $ dune build @use/unused-libs
+  File "use/dune", lines 1-3, characters 0-41:
+  1 | (executable
+  2 |  (libraries bar)
+  3 |  (name use))
+  Error: Unused libraries:
+  - foo
+  [1]