Nested slices are now detected, converted into prefabs, and the top-level prefab will get linked to the nested prefabs with the proper number of instances. However, the nested prefabs won't retain any of the slice override values or parent entity hierarchy. That will (hopefully) be added in a separate PR.
This also adds support for better relative source paths for nested prefabs. To support this, the tool now needs to connect/disconnect with the AssetProcessor to be able to turn a slice asset ID into a relative source path, so that nested templates can be looked up and converted correctly.