test: observational baseline for an unreferenced transitive library dep

An intermediate library [libB] declares [(libraries libA)] but its
own source does not reference any module of [libA]. The consumer
[main] uses [libB] and so transitively gains [libA] in its
compilation context.

Today's compile rules carry a glob over every transitively-reached
library's public-cmi directory, so editing any of [libA]'s modules
re-invalidates [main]. The test records the current rebuild count.

Adds a baseline that future per-module dependency tracking work
(ocaml#4572) can promote to a smaller number when the over-invalidation
is fixed.

Reproducer originally from @nojb:
ocaml#14116 (comment)

Signed-off-by: Robin Bate Boerop <me@robinbb.com>

Author: robinbb

test/blackbox-tests/test-cases/per-module-lib-deps/transitive-unreferenced-lib.t

@@ -0,0 +1,57 @@
+Baseline: an intermediate library [intermediate_lib] declares
+[(libraries dep_lib)] but its module does not actually reference
+any of [dep_lib]'s modules. The consumer [main] uses
+[intermediate_lib] and so transitively gains [dep_lib] in its
+compilation context.
+
+Today every consumer module declares a glob over each transitively-
+reached library's public-cmi directory, so editing
+[unreferenced_dep.ml] (which no source file references) re-
+invalidates [Main]. The test records the current rebuild count of
+[Main] when [unreferenced_dep.ml] is touched.
+
+This test is observational: a tighter dependency tracker that drops
+unreferenced libraries from compile rules' deps would lower the
+count.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.23)
+  > EOF
+
+  $ cat > dune <<EOF
+  > (library
+  >  (name dep_lib)
+  >  (wrapped false)
+  >  (modules unreferenced_dep referenced_dep))
+  > (library
+  >  (name intermediate_lib)
+  >  (wrapped false)
+  >  (modules intermediate_module)
+  >  (libraries dep_lib))
+  > (executable (name main) (modules main) (libraries intermediate_lib))
+  > EOF
+
+  $ cat > unreferenced_dep.ml <<EOF
+  > let x = 42
+  > EOF
+  $ cat > referenced_dep.ml <<EOF
+  > let x = 43
+  > EOF
+  $ cat > intermediate_module.ml <<EOF
+  > let x = 42
+  > EOF
+  $ cat > main.ml <<EOF
+  > let _ = Intermediate_module.x
+  > EOF
+
+  $ dune build @check
+
+Edit [unreferenced_dep.ml]. Neither [main.ml] nor
+[intermediate_module.ml] references [Unreferenced_dep] or any
+other [dep_lib] module, so a tighter filter could leave [Main]
+untouched. Today [Main] is rebuilt:
+
+  $ echo > unreferenced_dep.ml
+  $ dune build @check
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("dune__exe__Main"))] | length'
+  2