test: regression guard for [(root_module ...)] + [(libraries ...)]

Adds [test-cases/per-module-lib-deps/root-module-tight-deps.t].
Asserts that a library declaring [(root_module ...)] alongside
[(libraries ...)] builds correctly when its modules reference the
dep library through the Root alias.

Root modules carry their own dep-rule shape: dune synthesises the
[.d] file for the Root module (no [.all-deps], to avoid the cycle
that would arise from a Root that references itself transitively).
Any future change to inter-library dep handling that misses the
synthesised-[.d] path would regress this scenario.

Forward-looking guard. The build path on current main uses a
broad-glob over the dep library's objdir, so the test passes
trivially today. The synthesised-[.d] path is still exercised by
rule generation, and that is what the test pins.

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

Author: robinbb

test/blackbox-tests/test-cases/per-module-lib-deps/root-module-tight-deps.t

@@ -0,0 +1,42 @@
+A library declaring [(root_module ...)] alongside [(libraries
+...)] must build correctly when its modules reference the dep
+library through the Root alias.
+
+Root modules carry their own dep-rule shape: dune synthesises the
+[.d] file for the Root module (no [.all-deps], to avoid the cycle
+that would arise from a Root that references itself transitively).
+Any future change to inter-library dep handling that misses the
+synthesised-[.d] path would regress this scenario.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.23)
+  > EOF
+
+[dep_lib]: an unwrapped dep library.
+
+  $ mkdir dep_lib
+  $ cat > dep_lib/dune <<EOF
+  > (library (name dep_lib))
+  > EOF
+  $ cat > dep_lib/dep_lib.ml <<EOF
+  > let v = 1
+  > EOF
+  $ cat > dep_lib/dep_lib.mli <<EOF
+  > val v : int
+  > EOF
+
+[consumer_lib]: declares [(root_module root) (libraries dep_lib)]
+and references [Dep_lib] through [Root].
+
+  $ mkdir consumer_lib
+  $ cat > consumer_lib/dune <<EOF
+  > (library
+  >  (name consumer_lib)
+  >  (libraries dep_lib)
+  >  (root_module root))
+  > EOF
+  $ cat > consumer_lib/m.ml <<EOF
+  > let _ = Root.Dep_lib.v
+  > EOF
+
+  $ dune build @check