[Bam.Core] Added a new field attribute, Bam.Core.SiblingModulesAttribute. This can be used, in place of Bam.Core.RequiredModulesAttribute, to indicate that other modules should be considered as siblings, so if the primary module is a top level (i.e. in the package bam is started in), then so are it's siblings. Allows other modules to built alongside, but not be forced to have an explicit requirement.