Add repro case for #1645 (#6021)

Signed-off-by: Etienne Millon <me@emillon.org>

test/blackbox-tests/test-cases/github1645.t

@@ -0,0 +1,43 @@
+We create two libraries `l.one` and `l.two` with a conflicting module.
+They build fine, are not co-linkable, but documentation should be able to be
+built. See #1645.
+
+  $ cat > dune-project << EOF
+  > (lang dune 1.0)
+  > (package (name l))
+  > EOF
+
+  $ mkdir one
+  $ cat > one/dune << EOF
+  > (library
+  >  (name l_one)
+  >  (public_name l.one)
+  >  (wrapped false))
+  > EOF
+  $ touch one/module.ml
+
+  $ mkdir two
+  $ cat > two/dune << EOF
+  > (library
+  >  (name l_two)
+  >  (public_name l.two)
+  >  (wrapped false))
+  > EOF
+  $ touch two/module.ml
+
+  $ dune build @install
+  $ dune build @doc
+  Error: Multiple rules generated for
+  _build/default/_doc/_html/l/Module/.dummy:
+  - <internal location>
+  - <internal location>
+  -> required by alias _doc/_html/l/doc
+  -> required by alias doc
+  Error: Multiple rules generated for
+  _build/default/_doc/_odocls/l/module.odocl:
+  - <internal location>
+  - <internal location>
+  -> required by _build/default/_doc/_html/l/index.html
+  -> required by alias _doc/_html/l/doc
+  -> required by alias doc
+  [1]