test: document transparent alias and library recompilation (#4572)

Add baseline test verifying that incremental builds succeed when a module
re-exports a library via a transparent alias (module M = Mylib) and the
library's interface changes. Regression reported by @Alizter.

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

Author: robinbb

test/blackbox-tests/test-cases/per-module-lib-deps/transparent-alias.t

@@ -0,0 +1,51 @@
+Baseline: transparent module aliases and library dependency recompilation.
+
+When a module re-exports a library via a transparent alias (module M = Mylib),
+consumers that use the alias must be recompiled when the library changes.
+
+See: https://github.com/ocaml/dune/issues/4572
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+  $ mkdir lib
+  $ cat > lib/dune <<EOF
+  > (library (name mylib))
+  > EOF
+  $ cat > lib/mylib.ml <<EOF
+  > let v = 42
+  > EOF
+  $ cat > lib/mylib.mli <<EOF
+  > val v : int
+  > EOF
+
+  $ cat > dune <<EOF
+  > (executable (name main) (libraries mylib))
+  > EOF
+  $ cat > re.ml <<EOF
+  > module M = Mylib
+  > EOF
+  $ cat > re.mli <<EOF
+  > module M = Mylib
+  > EOF
+  $ cat > main.ml <<EOF
+  > let () = print_int Re.M.v
+  > EOF
+
+  $ dune build ./main.exe
+
+Change mylib's interface:
+
+  $ cat > lib/mylib.mli <<EOF
+  > val v : int
+  > val w : string
+  > EOF
+  $ cat > lib/mylib.ml <<EOF
+  > let v = 42
+  > let w = ""
+  > EOF
+
+The incremental build must succeed:
+
+  $ dune build ./main.exe