Package conflicts are ignored when dune-projects contains multiple conflicts

[gridbugs]

For example, if you have a dune-project with a single conflict it will be respected:

(lang dune 3.17)
(package
 (name foo)
 (depends bar)
 (conflicts bar))

Dune is unable to find a solution for the above package because it both depends on and conflicts with the package bar. However if we add an additional conflict dune will be able to find a solution:

(lang dune 3.17)
(package
 (name foo)
 (depends bar)
 (conflicts bar baz))

Dune will now happily choose bar as part of the package solution despite it appearing in conflicts. Note that if bar is repeated in conflicts rather than introducing a new conflicting package (ie. (conflicts bar bar)) then dune works as expected.

Repro: #11266