This test demonstrates a local package that's in the same conflict-class of a
dependency.

  $ mkrepo
  $ add_mock_repo_if_needed

  $ mkpkg conflict-class

  $ mkpkg bar <<EOF
  > conflict-class: "ccc"
  > EOF

Local conflict class defined in a local package:

  $ cat >foo.opam <<EOF
  > opam-version: "2.0"
  > depends: [ "bar" ]
  > conflict-class: "ccc"
  > EOF

  $ cat >x.opam <<EOF
  > opam-version: "2.0"
  > depends: "foo"
  > EOF

  $ make_dune_project 3.11

  $ dune_pkg_lock_normalized
  Error:
  Unable to solve dependencies while generating lock directory: dune.lock
  
  Couldn't solve the package dependency formula.
  Selected candidates: foo.dev x.dev foo&x
  - bar -> (problem)
      Rejected candidates:
        bar.0.0.1: In same conflict class (ccc) as foo
  [1]

Now the conflict class comes from the opam repository

  $ mkpkg foo <<EOF
  > depends: [ "bar" ]
  > conflict-class: "ccc"
  > EOF

  $ rm foo.opam

  $ dune_pkg_lock_normalized
  Error:
  Unable to solve dependencies while generating lock directory: dune.lock
  
  Couldn't solve the package dependency formula.
  Selected candidates: foo.0.0.1 x.dev
  - bar -> (problem)
      Rejected candidates:
        bar.0.0.1: In same conflict class (ccc) as foo
  [1]
