unreal_asset: add support for NON_OUTER_PACKAGE_IMPORT

[jariz]

See #105

[localcc]

Needs a write implementation and tests for an affected asset added.

[jariz]

Unfortunately it seems that the asset I was working with wasn't actually using this feature.
I have no idea how a import can reference a a external asset in any other way though, FModel gathers the following from it:

      "MediaList": [
        {
          "ObjectName": "AkMediaAsset'267782866'",
          "ObjectPath": "PenDriverPro/Content/WwiseAudio/Media/267782866.0"
        }

which clearly looks like a import from a different asset to me.

Anyway, if someone can deliver such an example asset, I'd be fine with writing the test and a write implementation to not let my effort go to waste completely, but until then, can't really finish this I guess.

[localcc]

Import can reference another asset through the import table iirc, so the field should just be a package index that is negative (meaning it is an import). And then the import table should contain the asset path that it references.