[FIRRTL] LowerToHW discards private modules

[fabianschuiki]

The LowerToHW pass currently discards private modules that are not instantiated. This is problematic if the modules contain contracts that we would like to verify later on. It would be nice if LowerToHW would simply lower the existing modules, and if we used SymbolDCE or a similar pass later on to discard unused modules.