[Design] Changes to support inheritance of tag helpers from view start

Usually the `TagHelperRegistrationVisitor` takes care of duplicates via a `HashSet`. as it is right now, any "found" `TagHelperDescriptors` could potentially be duplicates of those found in `ViewStart` and would then later be pulled apart by the registration process inside of the `TagHelperDescriptorProvider`.  Not sure I like the reliance on the `TagHelperDescriptorProvider` to remove dups.

Might be better to force a `HashSet` here so we don't have dups at this layer.