[native_assets_builder] Output a list of dependencies

[dcharkes]

Currently, NativeAssetsBuildRunner.build returns a Future<List<Asset>>.

However, for a flutter_tools Target building native assets, we need the list of dependencies in addition to the list of assets.

We do have this for individual builds in the BuildOutut from the native_assets_cli, but we don't provide an aggregate for this the native_assets_builder.

We should add some API that we can extend later. (Most likely we'd also want some way to display the errors in flutter_tools on builds that fail rather than rely on only streaming logs in the context of #33.)