Modifications to support providing tag descriptors from derived hosts

Don't do a `Distinct` here, it doesn't buy you anything, the `TagHelperDescriptorProvider` ensures no duplicates.  In my previous comments I was just saying that waiting for the `TagHelperDescriptorProvider` to remove duplicate descriptors cause unneeded work at the layers above it, ex: `TagHelperRegistrationVisitor` loading same assembly multiple times etc.