[FIRRTL] LowerToHW discards private modules

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.

Start by locating the LowerToHW pass and tracing where private, uninstantiated modules are removed. Compare its behavior with SymbolDCE or a similar later pass, using existing LowerToHW tests if present. Done means LowerToHW lowers existing private modules without discarding them, leaving unused-module elimination to a later pass.

Written by the indexing model from the issue text.