Adjust v15 usage to use exposed extrinsic, Call & Event fields

[jacogr]

After #5720

This replaces the alias & type registration in PortableRegistry

(This is an investigation, since the types are parsed here, the full metadata is not available - so we would most probably need to keep with the doubling-up of applied logic - or alternatively not do it all all in PortableRegistry and move it elsewhere)