test: demonstrate failure to digest installed dir target dependency

doc/changes/fixed/13045.md

@@ -0,0 +1,2 @@
+- Fix failure to digest installed directory targets, allowing them to be used
+  as dependencies to other rules. (#13045, @anmonteiro)

src/dune_digest/cached_digest.ml

@@ -375,7 +375,7 @@ let remove path =
 
 module Untracked = struct
   let source_or_external_file path =
-    peek_or_refresh_file ~allow_dirs:false (Path.outside_build_dir path)
+    peek_or_refresh_file ~allow_dirs:true (Path.outside_build_dir path)
   ;;
 
   let invalidate_cached_timestamp path =

test/blackbox-tests/test-cases/directory-targets/depend-on-installed-dir-target.t

@@ -0,0 +1,55 @@
+Test installed directory targets may be depended on
+
+  $ mkdir lib app prefix
+  $ cat > lib/dune-project <<EOF
+  > (lang dune 3.21)
+  > (package (name foo))
+  > (using directory-targets 0.1)
+  > EOF
+
+  $ cat > lib/dune <<EOF
+  > (rule (target (dir some_dir))
+  >  (action
+  >   (progn (system "mkdir %{target}")
+  >    (system "echo hello from file inside dir target > %{target}/inside-dir-target.txt"))))
+  > (library (public_name foo) (modes byte))
+  > (install (section lib) (dirs some_dir))
+  > EOF
+
+  $ dune build --root lib
+  Entering directory 'lib'
+  Leaving directory 'lib'
+
+  $ cat lib/_build/default/foo.install
+  lib: [
+    "_build/install/default/lib/foo/META"
+    "_build/install/default/lib/foo/dune-package"
+    "_build/install/default/lib/foo/foo.cma"
+    "_build/install/default/lib/foo/foo.cmi"
+    "_build/install/default/lib/foo/foo.cmt"
+    "_build/install/default/lib/foo/foo.ml"
+    "_build/install/default/lib/foo/some_dir/inside-dir-target.txt" {"some_dir/inside-dir-target.txt"}
+  ]
+
+  $ cat lib/_build/install/default/lib/foo/dune-package | grep some_dir
+   (lib (META dune-package foo.cma foo.cmi foo.cmt foo.ml (dir some_dir))))
+
+  $ dune install --root lib --prefix $PWD/prefix
+
+  $ cat > app/dune-project <<EOF
+  > (lang dune 3.21)
+  > (package (name app))
+  > EOF
+  $ cat > app/dune <<EOF
+  > (rule
+  >  (target hello.txt)
+  >  (deps %{lib:foo:some_dir})
+  >  (action (system "cp %{deps}/inside-dir-target.txt %{target}")))
+  > EOF
+
+  $ OCAMLPATH=$PWD/prefix/lib/:$OCAMLPATH dune build --root app hello.txt
+  Entering directory 'app'
+  Leaving directory 'app'
+
+  $ cat app/_build/default/hello.txt
+  hello from file inside dir target