[unreal_asset] Support for import package names

[jariz]

I'm working with a asset that has a package name that I want to update, which apparently is a feature that is not available in all versions of UE4.

Here's how CUE4Parse handles this:
https://github.com/FabianFG/CUE4Parse/blob/e05d309a9352cf8e6b6d1a78bfb9e009c5f41323/CUE4Parse/UE4/Objects/UObject/ObjectResource.cs#L288-L291

I can probably work around this by messing with the fnames map but it would be nice to have a more formal way of getting this FName.