[9.x] Map integer parameter to parameter name when resolving binding field

[ksassnowski]

Fixes #42541

First of all, I'm really sorry about all the issues the initial PR has caused 😞

Summary

This PR reverts most of the changes of the original PR (#42425) while still fixing the bug described in the PR. The only method that was changed was the bindingFieldFor method of the Route class. All changes to ImplicitRouteBinding and RouteUri were reverted.

To fix the bug described in the original PR, we now resolve the parameter index to the parameter name by looking it up in the route's parameterNames array.

public function bindingFieldFor($parameter)
{
    if (is_int($parameter))  {
        $parameter = $this->parameterNames()[$parameter];
    }

    return $this->bindingFields[$parameter] ?? null;
}

This means that we no longer have to change the structure of the bindingFields array itself like in the original PR. The bug described in the related issue (#42541) was due to the fact that the ResourceRouteRegistrar also filled the route's binding fields with null values, just like my change in the original PR. The problem with that is that both of these implementations used null to signal the exact opposite of each other. My PR used null to mean "don't scope this parameter", whereas the ResourceRouteRegistrar used it to mean "scope this parameter but default to the model's route key name".

I left in all the additional tests I added since all the problems that got reported due to my original PR were not covered by any existing tests.

[taylorotwell]

Thanks!

[mortenscheel]

FYI I think this PR introduced a breaking change in a rather exotic edge case. I posted a bug report (#42605), but I closed it myself when I realized that it only happens if you ask the route() helper to turn models into query params. Which is obviously a mistake, but up until v9.14 it produced valid URL's. Now it produces 500 errors.