Signed-off-by: Ryan Gonzalez <[email protected]>

This upgrades the progress output quite a bit, largely thanks to the
rich CLI library:

- There's a pretty animation at the start of the line!
- Instead of printing the last package interacted with (which made no
  sense), show as many of the currently-processing packages as can fit
  on a single line.
- Show live download progress for large downloads, so any that are
  holding up generation don't appears to be hanging.

In addition, this adds a hidden CLI flag, --traceback-on-interrupt, that
prints a full traceback from every active package coroutine on Ctrl-C,
which was useful while debugging this (and probably will be in the
future!)

Signed-off-by: Ryan Gonzalez <[email protected]>