[StaticWebAssets] Validate that no two assets end up in the same final path instead of checking they all have the same content root

[javiercn]

Now that we support scoped css we need to be able to accept source items that come from different content roots. For that reason we need to change the check that we do that requires all items coming from the same content root, for one that ensures that they don't end up in the same final path.