Allow children from import sources with the same package scope

[rmorshea]

Discussed in #534

If the import sources of two web modules are different, but have the same package scope, then they should be allow to be children of each other. There may be a more general way of indicating that two import sources are compatible with each other. Since the main goal of restricting what what import sources can or can't be children of each other is to ensure that they are using the same instance of React, maybe import sources can indicate this somehow, either through their name or metadata attached to their import source dict.

[Archmonger]

Closed in favor of #1262