support multiple base packages in Android modules

[RBusarow]

Some Android projects choose to have different base packages for different source sets -- especially androidTest. For instance, they might follow a pattern like:

source set	base package
androidTest	com.example.app.test
debug	com.example.app.debug
main	com.example.app

This is done by adding a small manifest file with just the custom package to the inheriting source set. It will override any upstream (inherited) package definition.

For the sake of simplicity, ModuleCheck currently only supports a single base package per module -- throwing an exception if it finds more. I never intended for that to be the final policy, but some other infrastructure was missing so a more rigorous solution would have been difficult.

Instead of just having a single androidPackage property per project, they should be keyed by SourceSet. For lookups, if a base package is undefined for a specific source set, like debug, then look to its upstream source set(s) (like main).