Generator improvements, Part 2

Following !165 (merged) here are some attempts to:

• print which part is being generated (maybe we now need a "be more quiet" mode): I find that most valuable
• print a human-friendly error when a specified part doesn't exist
• (optional) I added a --dry-run parameter (same parameter name exists in rsync) that skips generation of parts (can be also used as a hack to list existing parts, though it would probably be better to add listing as an explicit option; I found it helpful for debugging these changes, but it's not essential to keep it)
• it is now possible to only specify --package without --library
• (Inductors should ideally be modified to provide listing of available parts.)

I don't care if this gets postponed.

@kliment1