[4.x] Add validation to prevent duplicate field handles

[duncanmcclean]

This pull request implements a validation rule to prevent adding multiple fields to a blueprint with the same handle.

Previous to this PR, blueprints & fieldsets behaved differently to duplicate field handles:

• When you saved a blueprint where multiple fields share a handle, only one of the fields would actually be saved.
• When you saved a fieldset where multiple fields share a handle, both of the fields would be saved, causing issues like that seen in Error when searching in the Link a single field box #9333.

Now, with this PR, when you attempt to save or update a field using the same handle as another field, you'll see a validation error:

The validation will check both actual fields in the blueprint as well as any fields imported via fieldsets.

Closes #9333.

[jasonvarga]

We already have validation on the blueprint level:

cms/src/Fields/Blueprint.php

Lines 613 to 622 in ec3c2a4

	public function validateUniqueHandles()
	{
	$fields = $this->fieldsCache ?? new Fields($this->tabs()->map->fields()->flatMap->items());
	$handles = $fields->resolveFields()->map->handle();
	if ($field = $handles->duplicates()->first()) {
	throw new DuplicateFieldException($field, $this);
	}
	}

But, at some point I guess something changed and it doesn't work quite right. It looks like by the time it gets to line 619, the duplicates are already gone, so the exception doesn't get thrown. (Line 615 changed as part of #7746. It was probably that.)

Regardless, even if it worked, the validation error only happens when you save the entire blueprint.

This PR is a nice addition because it'll prevent you from putting a duplicate into the field stack in the first place. 👍