Relax prerequisites for java.base-jmod target

The top level target "java.base-jmod" currently has all other jmod targets on the prerequisites list. This is because we store a checksum for every non upgradeable module in java.base and most of the modules aren't upgradeable. But, since we do have upgradeable modules, those shouldn't be on the prerequisites list for java.base-jmod.

Fixing this won't impact the build much, but certainly won't hurt either.