[5.8] Fix top level wildcard validation for 'distinct'

[me-shaon]

When someone writes validation rules like this:

$array = [
    ['id' => 1],
    ['id' => 1],
];
$rules = [
    '*' => 'array',
    '*.id' => 'distinct',
];
Validator::make($array, $rules)->validate();

it doesn't work as expected.

Why?

The implicitAttributes array in Validator class stores the mapping of 'unparsed' rule to 'parsed' rule.
For the given rule, it is like:

[
  "*" => [
    0 => 0
    1 => 1
  ],
  "*.id" => [
    0 => "0.id"
    1 => "1.id"
  ]
]

So, when this array is searched to get the key of "0.id", it finds * instead of *.id because of loose comparison match between "0.id" and 0.
This scenario can be avoided if all the parsed rule are stored as string, which I did in this PR.
I've also added tests to verify the correctness of my solution.

This PR will fix #29190