Builders need some way of knowing the "root" package.

[jakemac53]

It could be that you simply supply this when you create them, in which case this issue can just be closed, but its a common enough case that we might want something more general.

[jakemac53]

To be clear, this is because builders can only output assets in the root package, but they can have primary inputs from other packages. When processing a primary input for another package, they will need to output files in the root one but don't know what that is.

[natebosch]

I think we've got this one covered.