Reproduction case for ignored dune.lock (#11115)

Thanks to @mefyl for reporting the case and @gridbugs for isolating the
issue.

Signed-off-by: Marek Kubica <marek@tarides.com>

test/blackbox-tests/test-cases/pkg/ignored-dune-lock.t

@@ -0,0 +1,55 @@
+Test that shows what happens when dune.lock is ignored.
+
+  $ . ./helpers.sh
+
+  $ make_lockdir
+  $ cat >dune.lock/test.pkg <<EOF
+  > (version 0.0.1)
+  > (build
+  >  (progn
+  >   (run touch foo.ml)
+  >   (patch foo.patch)))
+  > EOF
+  $ mkdir dune.lock/test.files
+  $ cat > dune.lock/test.files/foo.patch <<EOF
+  > diff --git a/foo.ml b/foo.ml
+  > index b69a69a5a..ea988f6bd 100644
+  > --- a/foo.ml
+  > +++ b/foo.ml
+  > EOF
+
+  $ mkdir src
+  $ cat > src/dune <<EOF
+  > (executable
+  >  (name foo))
+  > EOF
+  $ cat > src/foo.ml <<EOF
+  > let () = ()
+  > EOF
+  > cat > dune-project <<EOF
+  > (lang dune 3.16)
+  > EOF
+
+Building test works when the dune.lock is visible to dune.
+
+  $ build_pkg test
+
+Now the project is changed to only include src (which effectively ignores
+dune.lock):
+
+  $ cat > dune <<EOF
+  > (dirs src)
+  > EOF
+
+Building fails as the patch cannot be found anymore
+
+  $ build_pkg test 2>&1 | sed 's|\.sandbox/[a-f0-9]*/|.sandbox/<hash>/|'
+  Error:
+  _build/.sandbox/<hash>/_private/default/.pkg/test/source/foo.patch:
+  No such file or directory
+  -> required by _build/_private/default/.pkg/test/target
+
+And the backage cannot be shown:
+
+  $ show_pkg test
+