Script Loader: Ensure delayed members of script bundles are handled properly

[westonruter]

A dependency bundle (where src is false) is treated as a blocking dependency. Nevertheless, the bundle may consist of delayed dependents.

This may not be fully fleshed out in the implementation, namely if a bundle has an async dependent but the bundle whole is a dependency of a defer dependent. We should make sure the async gets converted to defer here.

Originally posted by @westonruter in #63 (comment)