dune should not fail detecting rule dependencies when the rule is not needed #3252
Description
Issue encountered in ocaml/opam-repository#15967
Here is minimal test-case:
$ cat dune
(rule
(targets testfile)
(deps %{bin:atdgen})
(action (echo "test")))
$ dune build @install
File "dune", line 3, characters 9-20:
3 | (deps %{bin:atdgen})
^^^^^^^^^^^
Error: Program atdgen not found in the tree or in PATH
(context: default)
$ dune --version
2.4.0
cc @rgrinberg