Make nested IsInstantiables with Data in them work (#2761)

* Add unit test for Issue 2760

* checkpoint: Fix for nested instance

* remove comments about stuff not working

* make the test check the output a little more

* relax the requirement on returning empty ioMap

* Update core/src/main/scala/chisel3/experimental/hierarchy/core/Lookupable.scala

* Update core/src/main/scala/chisel3/Data.scala

* Update core/src/main/scala/chisel3/experimental/hierarchy/core/Lookupable.scala

Co-authored-by: Jack Koenig <koenig@sifive.com>

* Update src/test/scala/chiselTests/experimental/hierarchy/InstanceSpec.scala

Co-authored-by: Jack Koenig <koenig@sifive.com>

* Update core/src/main/scala/chisel3/experimental/hierarchy/core/Lookupable.scala

* Add another unit test which unfortunately still passes

* Update core/src/main/scala/chisel3/Data.scala

* Update src/test/scala/chiselTests/experimental/hierarchy/InstanceSpec.scala

Co-authored-by: Jack Koenig <koenig@sifive.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>