cleanup ModuleSymbol.getSimpleName

ModuleSymbol.getSimpleName derives its name from the qualified name, which is the more typical name to use.

Either we should change ModuleSymbol.name to be the simpleName ... which would require lots of use sites to change to use the flat/full name, or (simpler) we should change the inline code in MS.getSimpleName to use Convert.getSimpleName. That method is functionally equivalent, but the docs and parameter names indicate that it is (just) for classes, and not for any qualified name.