[native_assets_cli][native_assets_builder] Dry run dependencies & dry run caching

[dcharkes]

Dry runs don't have to report dependencies right now. The rationale being that no assets are being built, and if for example some native compilation is happening a dry run should not be concerned about

However, we could consider the 'dependencies' of a dry run to be the files that are required to come up with the list of assets. Then we could cache dry runs.

I'm not entirely sure what this will look like once we nest dependencies inside assets though:

• https://github.com/dart-lang/native/issues/1208