[5.7] Improve nested rules in validated data

[staudenmeir]

(re-submit of #23703 for 5.7)

#20974 fixed an issue with array rules (users.*.name) when returning the validated data.
Nested rules now get reduced to the first key.

However, this wouldn't be necessary for rules without an asterisk:

$data = Validator::make(['a' => ['b' => 'x', 'c' => 'y']], ['a.b' => 'required'])->validate();
// Expected: ['a' => ['b' => 'x']]
// Actual: ['a' => ['b' => 'x', 'c' => 'y']]

This PR differentiates between rules having and not having an asterisk. Only the former get reduced.

For the request validation this is easy to achieve because Request::only() already supports nested keys.

Validator::validate() however uses Collection::only() which doesn't support nested keys:

$data = collect(['a' => ['b' => 'x']])->only('a.b')->all();
// Expected: ['a' => ['b' => 'x']]
// Actual: []

So we have to use data_get() and Arr::set() (as used in Request::only()) to bring support for nested keys to Validator.

Fixes #23612.

[morloderex]

Isn't it better to just add support for nested keys inside the collection?