[RTL/SV] Move UnpackedArrayType to SV dialect

`UnpackedArrayType` in the RTL dialect is more of a syntactic/behavioral construct that only makes sense in the SV world. We should move it over there.

Unfortunately, this is trickier than it sounds because the RTL dialect library currently does not depend on the SV dialect library (SV depends on RTL and we want lib dependencies to be a DAG) and there is various checking and other logic in RTL land that needs to be aware of this type.

That said, it is fixable and we should detangle this.

Start by locating UnpackedArrayType in the RTL dialect and the SV dialect library, then trace the RTL checking and related logic that recognizes the type. The work is complete when the type belongs to SV, the library dependency remains a DAG, and the affected RTL logic still handles it correctly.

Written by the indexing model from the issue text.